| **Summary of Offsetting Assets and Liabilities** | 12 Months Ended                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                        |
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| **Offsetting [Abstract]**                        |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                   |
| Summary of Offsetting Assets and Liabilities     | Summary of Offsetting Assets and Liabilities The Company enters into derivative transactions with several approved counterparties. The Company’s derivative transactions are generally governed by ISDA or MSFTA Master Agreements which provide for legally enforceable set-off and close-out netting in the event of default or bankruptcy of the Company’s counterparties.  The Company’s ISDA and MSFTA Master Agreements generally include provisions which require both the pledging and accepting of collateral in connection with its derivative transactions. These provisions have the effect of securing each party’s position to the extent of collateral held.  The following tables summarize the effect of master netting arrangements on the Company’s financial position in the normal course of business and in the event of default or bankruptcy of the Company’s counterparties:   December 31, 2014    Gross fair value not offset      in balance sheets    Gross fair value of Financial Cash collateral NetFinancial instruments: recognized assets/liabilities (1) instruments received/(pledged) fair valueDerivative instruments (assets) (2) $32,895 $(32,595) $279 $21Derivative instruments (liabilities) (3) 140,655 (32,595) (105,929) 2,131   December 31, 2013    Gross fair value not offset      in balance sheets    Gross fair value of Financial Cash collateral NetFinancial instruments: recognized assets/liabilities (1) instruments received/(pledged) fair valueDerivative instruments (assets) (2) $25,250 $(25,023) $— $227Derivative instruments (liabilities) (3) 171,387 (25,023) (143,540) 2,824 (1) The gross fair value of derivative instruments are not netted against offsetting liabilities for presentation on the consolidated balance sheets.(2) The estimated fair value of derivative instrument assets is reported in other assets in the consolidated balance sheets.  Derivative transactions entered into under ISDA master agreements include income and expense accruals.(3) The estimated fair value of derivative instrument liabilities is reported in other liabilities in the consolidated balance sheets.  Derivative transactions entered into under ISDA master agreements include income and expense accruals. |

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| - Definition
The entire disclosure of derivative and other financial assets and liabilities that are subject to offsetting, including master netting arrangements.
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No definition available.
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| - Details**Name:**us-gaap_OffsettingAbstract**Namespace Prefix:**us-gaap_**Data Type:**xbrli:stringItemType**Balance Type:**na**Period Type:**duration |