| Derivative Instruments and Hedging Activities Credit Risk - Counterparty Exposure (Details) - USD ($) $ in Thousands | Mar. 31, 2016 | Dec. 31, 2015 | 
|---|---|---|
| Credit Derivatives [Line Items] | ||
| Collateral Securities Repledged, Delivered, or Used | $ (2,220) | |
| Collateral Already Posted, Aggregate Fair Value | 232,054 | |
| Derivative Financial Instruments, Assets [Member] | ||
| Credit Derivatives [Line Items] | ||
| Derivative, Fair Value, Net | 197,847 | $ 271,509 | 
| Derivative Financial Instruments, Liabilities [Member] | ||
| Credit Derivatives [Line Items] | ||
| Derivative, Fair Value, Net | $ (77,038) | $ (7,285) | 
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- Definition The aggregate fair value of assets that are already posted, at the end of the reporting period, as collateral for derivative instruments with credit-risk-related contingent features. Reference 1: http://www.xbrl.org/2003/role/presentationRef 
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- Definition The carrying value as of the balance sheet date of securities received as collateral from a counterparty that were re-pledged, sold, delivered or otherwise reused by the entity. Reference 1: http://www.xbrl.org/2003/role/presentationRef 
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- Definition Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table. No definition available. 
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- Definition Fair value of the assets less the liabilities of a derivative or group of derivatives. Reference 1: http://www.xbrl.org/2003/role/presentationRef 
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