Repurchase Agreements - Additional Information (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended	12 Months Ended
Mar. 31, 2015 Person	Dec. 31, 2014 Person
Banking And Thrift [Abstract]
Number of counterparties in repurchase agreements	28	25
Weighted average contractual maturity of the repurchase agreements outstanding	1 month 6 days	1 month
Fair value of assets pledged as collateral under repurchase agreements	$ 15,916,728	$ 16,575,106

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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