| **Segment Information (Tables)**                     | 3 Months Ended                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                               |
|------------------------------------------------------|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
||
| **Segment Information [Abstract]**                   |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                         |
| Company's Segment Information, Continuing Operations |  

						 

						 

						 

						 

						 

						 

						 

						 

						 

						 

						 

						 

						 

						 

						 

						 

						 

						 

						 

 

						 

						 

						 

						 

						 

						 

						 

						 

						 

						 

						 

						 

						 

						 

						 

						 

						 

						 

						 

 

						 

						 

Three Months Ended

						 

Three Months Ended

 

						 

						 

March 31, 2014

						 

March 31, 2013

 

						 

						 

Regulated

						 

Other

						 

Consolidated

						 

Regulated

						 

Other

						 

Consolidated

 

Operating revenues

						 

$
178,199 				
						 

$
4,473 				
						 

$
182,672 				
						 

$
174,405 				
						 

$
4,147 				
						 

$
178,552 				
 

Operations and maintenance expense

						 

						 
68,456 				
						 

						 
3,230 				
						 

						 
71,686 				
						 

						 
65,410 				
						 

						 
2,384 				
						 

						 
67,794 				
 

Depreciation

						 

						 
30,881 				
						 

						 
100 				
						 

						 
30,981 				
						 

						 
29,510 				
						 

						 
(465)				
						 

						 
29,045 				
 

Operating income

						 

						 
66,419 				
						 

						 
351 				
						 

						 
66,770 				
						 

						 
65,550 				
						 

						 
1,388 				
						 

						 
66,938 				
 

Interest expense, net of allowance for funds used during construction

						 

						 
16,804 				
						 

						 
1,339 				
						 

						 
18,143 				
						 

						 
16,951 				
						 

						 
1,772 				
						 

						 
18,723 				
 

Income tax expense (benefit)

						 

						 
6,087 				
						 

						 
(895)				
						 

						 
5,192 				
						 

						 
7,352 				
						 

						 
(565)				
						 

						 
6,787 				
 

Income (loss) from continuing operations 

						 

						 
43,620 				
						 

						 
(1,219)				
						 

						 
42,401 				
						 

						 
41,299 				
						 

						 
(435)				
						 

						 
40,864 				
 

Capital expenditures

						 

						 
59,154 				
						 

						 
665 				
						 

						 
59,819 				
						 

						 
58,872 				
						 

						 
213 				
						 

						 
59,085 				
 

						 

						 

						 

						 

						 

						 

						 

						 

						 

						 

						 

						 

						 

						 

						 

						 

						 

						 

						 

 

						 

						 

						 

						 

						 

						 

						 

						 

						 

						 

						 

						 

						 

						 

						 

						 

						 

						 

						 

  |
| Company's Segment Information, Assets                |  

						 

						 

						 

						 

						 

						 

						 

 

						 

						 

March 31,

						 

December 31,

 

						 

						 

2014

						 

2013

 

Total assets:

						 

						 

						 

						 

						 

						 

 

   Regulated

						 

$
4,938,191 				
						 

$
4,893,573 				
 

   Other and eliminations

						 

						 
161,576 				
						 

						 
158,244 				
 

   Consolidated

						 

$
5,099,767 				
						 

$
5,051,817 				
 

						 

						 

						 

						 

						 

						 

						 

                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                      |

| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                 |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Tabular disclosure of all significant reconciling items in the reconciliation of total assets from reportable segments to the entity's consolidated assets.
+ References
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|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Tabular disclosure of all significant reconciling items in the reconciliation of total revenues from reportable segments to the entity's consolidated revenues.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 280 -SubTopic 10 -Section 50 -Paragraph 31 -URI http://asc.fasb.org/extlink&oid=6534315&loc=d3e8924-108599Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 280 -SubTopic 10 -Section 50 -Paragraph 30 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=6534315&loc=d3e8906-108599
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| X                                                                                                                                                            |
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