Repurchase Agreements - Additional Information (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
Dec. 31, 2014 Person	Dec. 31, 2013 Person
Banking And Thrift [Abstract]
Number of counterparties in repurchase agreements	25	25
Weighted average contractual maturity of the repurchase agreements outstanding (in months)	1 month	24 days
Fair value of assets pledged as collateral under repurchase agreements	$ 16,575,106	$ 17,088,392

Weighted average maturity (weighted by amount of the agreement) of repurchase agreements with the counterparty, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 08

-Paragraph m

-Subparagraph 1

-Article 4

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 235

-SubTopic 10

-Section S99

-Paragraph 1

-Subparagraph (SX 210.4-08.(m)(1)(i)(B)(iii))

-URI http://asc.fasb.org/extlink&oid=26873400&loc=d3e23780-122690

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