Derivative Instruments and Hedging Activities Interest Rate Sensitive Assets/Liabilities (Details) - USD ($) $ in Thousands  | 
Jun. 30, 2015  | 
Dec. 31, 2014  | 
|---|---|---|
| Derivatives, Fair Value [Line Items] | ||
| Available-for-sale securities, at fair value | $ 12,807,658 | $ 14,341,102 | 
| Interest-Only-Strip [Member] | ||
| Derivatives, Fair Value [Line Items] | ||
| Available-for-sale securities, at fair value | $ 49,278 | $ 55,686 | 
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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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