| Derivative Instruments and Hedging Activities Interest Rate Sensitive Assets/Liabilities (Details) - USD ($) $ in Thousands | Mar. 31, 2016 | Dec. 31, 2015 | 
|---|---|---|
| Derivatives, Fair Value [Line Items] | ||
| Available-for-sale securities, at fair value | $ 9,584,454 | $ 7,825,320 | 
| Interest-Only-Strip [Member] | ||
| Derivatives, Fair Value [Line Items] | ||
| Available-for-sale securities, at fair value | $ 39,071 | $ 42,931 | 
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- Definition Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading. 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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- Details 
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