Gross Gains and Losses from Investment Sales (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended
Mar. 31, 2015	Mar. 31, 2014
Investments Debt And Equity Securities [Abstract]
Gross gains	$ 16,510us-gaap_GainOnSaleOfInvestments	$ 7,436us-gaap_GainOnSaleOfInvestments
Gross losses	(57)us-gaap_LossOnSaleOfInvestments
Net gain	$ 16,453	$ 7,436

Net gain (loss) resulting from the sales of securitized, pay-through debt securities collateralized by real estate mortgage loans (mortgages).

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 942

-SubTopic 225

-Section S99

-Paragraph 1

-Subparagraph (SX 210.9-04.13(h))

-URI http://asc.fasb.org/extlink&oid=6879574&loc=d3e536633-122882

Name:	us-gaap_GainLossOnSalesOfMortgageBackedSecuritiesMBS
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration