| **Consolidated Statements Of Comprehensive Income (USD $)In Thousands, unless otherwise specified**                                                                                                     | 12 Months Ended                                                                                      |
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| **Consolidated Statements Of Comprehensive Income [Abstract]**                                                                                                                                          |                                                                                                 |   |   |
| Net income attributable to common shareholders                                                                                                                                                          | $ 233,239us-gaap_NetIncomeLoss                                                                       | $ 221,300us-gaap_NetIncomeLoss | $ 196,563us-gaap_NetIncomeLoss |
| **Other comprehensive income, net of tax:**                                                                                                                                                             |                                                                                                 |   |   |
| Unrealized holding gain on investments, net of tax expense of $104, $76, and $106 for the years ended December 31, 2014, 2013, and 2012, respectively                                                   | 193us-gaap_OtherComprehensiveIncomeUnrealizedHoldingGainLossOnSecuritiesArisingDuringPeriodNetOfTax  | 141us-gaap_OtherComprehensiveIncomeUnrealizedHoldingGainLossOnSecuritiesArisingDuringPeriodNetOfTax | 198us-gaap_OtherComprehensiveIncomeUnrealizedHoldingGainLossOnSecuritiesArisingDuringPeriodNetOfTax |
| Reclassification adjustment for loss (gain) reported in net income, net of tax (benefit) expense of $(134), $(49), and $182 for the twelve months ended December 31, 2014, 2013, and 2012, respectively | 249us-gaap_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIForSaleOfSecuritiesNetOfTax | 90us-gaap_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIForSaleOfSecuritiesNetOfTax | (339)us-gaap_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIForSaleOfSecuritiesNetOfTax |
| Comprehensive income                                                                                                                                                                                    | $ 233,681us-gaap_ComprehensiveIncomeNetOfTax                                                         | $ 221,531us-gaap_ComprehensiveIncomeNetOfTax | $ 196,422us-gaap_ComprehensiveIncomeNetOfTax |

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| - Definition
Amount after tax of increase (decrease) in equity from transactions and other events and circumstances from net income and other comprehensive income, attributable to parent entity. Excludes changes in equity resulting from investments by owners and distributions to owners.
+ References
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| - Definition
The portion of profit or loss for the period, net of income taxes, which is attributable to the parent.
+ References
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| - Definition
Amount after tax of reclassification adjustment from accumulated other comprehensive income for unrealized gain (loss) realized upon the sale of available-for-sale securities.
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| - Definition
Amount after tax, before reclassification adjustments, of unrealized holding gain (loss) on available-for-sale securities.
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