| **Investments (Tables)**                               | 6 Months Ended                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                    |
|--------------------------------------------------------|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
||
| **Investments, Debt and Equity Securities [Abstract]** |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                              |
| Schedule of Investments                                | At June 30, 2016 investments and restricted investments classified as available-for-sale securities were as follows: Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Fair ValueDebt securities:       U.S. Treasury securities$174,367 $197 $— $174,564Corporate debt securities20,086 78 1,152 19,012Preferred stock22,930 — 236 22,694 217,3832751,388216,270Restricted investments:       Money market fund5,653 — — 5,653 $223,036$275$1,388$221,923At December 31, 2015 investments and restricted investments classified as available-for-sale securities were as follows:  Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Fair ValueDebt securities:       U.S. Treasury securities$184,819 $— $79 $184,740Corporate debt securities7,273 — 981 6,292Preferred stock265 — 57 208 192,357—1,117191,240Restricted investments:       Guaranteed income fund7,072 — — 7,072 $199,429$—$1,117$198,312 |
| Continuous Unrealized Loss Position                    | The following table provides the debt securities and preferred stock unrealized loss position and related fair values:     As of June 30, 2016 As of December 31, 2015 Less Than 12 Months 12 Months or Greater Less Than 12 Months 12 Months or Greater Fair Value Unrealized Losses Fair Value Unrealized Losses Fair Value Unrealized Losses Fair Value Unrealized LossesDebt securities:               U.S. Treasury securities$— $— $— $— $184,740 $79 $— $—Corporate debt securities18,262 1,152 — — 6,292 981 — —Preferred stock22,694 236 — — 208 57 — — $40,956 $1,388 $— $— $191,240 $1,117 $— $—                                                                                                                                                                                                                                                                                                     |
| Contractual Maturities of Investments                  | The net carrying value and estimated fair value of investments and restricted investments classified as available-for-sale at June 30, 2016, by contractual maturity are shown in the following table.  Actual maturities may differ from contractual maturities because certain borrowers have the right to call or prepay obligations.  Amortized Cost Fair ValueDue in one year or less$174,367 $174,564Due after one year through five years19,987 18,929Due after ten years through fifteen years99 83 194,453 193,576Preferred stock 22,930 22,694Restricted investments5,653 5,653 $223,036 $221,923                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                           |

| X                                                                                                                                                                                                                                                                                                                                                                                                                                                         |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Tabular disclosure of fair value of investments in debt and equity securities in an unrealized loss position categorized neither as held-to-maturity nor trading securities.
+ References
No definition available.
+ Details**Name:**us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValueTableTextBlock**Namespace Prefix:**us-gaap_**Data Type:**nonnum:textBlockItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Tabular disclosure of available-for-sale securities which includes, but is not limited to, changes in the cost basis and fair value, fair value and gross unrealized gain (loss), fair values by type of security, contractual maturity and classification, amortized cost basis, contracts to acquire securities to be accounted for as available-for-sale, debt maturities, transfers to trading, change in net unrealized holding gain (loss) net of tax, continuous unrealized loss position fair value, aggregate losses qualitative disclosures, other than temporary impairment (OTTI) losses or other disclosures related to available for sale securities.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 320 -Section 50 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=6957658&loc=d3e62557-112803Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 7 -URI http://asc.fasb.org/extlink&oid=73527792&loc=d3e27337-111563Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 3 -URI http://asc.fasb.org/extlink&oid=73527792&loc=d3e27198-111563Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 320 -Section 50 -Paragraph 3 -URI http://asc.fasb.org/extlink&oid=6957658&loc=d3e62586-112803Reference 5: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=73527792&loc=d3e27161-111563Reference 6: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 9 -URI http://asc.fasb.org/extlink&oid=73527792&loc=d3e27357-111563Reference 7: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 6 -URI http://asc.fasb.org/extlink&oid=73527792&loc=d3e27290-111563
+ Details**Name:**us-gaap_AvailableForSaleSecuritiesTextBlock**Namespace Prefix:**us-gaap_**Data Type:**nonnum:textBlockItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                  |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Tabular disclosure of maturities of an entity's investments as well as any other information pertinent to the investments.
+ References
No definition available.
+ Details**Name:**us-gaap_InvestmentsClassifiedByContractualMaturityDateTableTextBlock**Namespace Prefix:**us-gaap_**Data Type:**nonnum:textBlockItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                    |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - References
No definition available.
+ Details**Name:**us-gaap_InvestmentsDebtAndEquitySecuritiesAbstract**Namespace Prefix:**us-gaap_**Data Type:**xbrli:stringItemType**Balance Type:**na**Period Type:**duration |