Mortgage-Backed Securities - By Maturity (Details) (USD $)	Mar. 31, 2015	Dec. 31, 2014
Mortgage Backed Securities [Abstract]
Greater than five years and less than ten years	$ 766,000	$ 967,000
Greater than or equal to ten years	1,675,855,000	1,548,204,000
Total mortgage-backed securities	$ 1,676,621,000	$ 1,549,171,000

Fair value of securitized, pay-through debt securities collateralized by real estate mortgage loans classified as available-for-sale.

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

-Publisher FASB

-Name Accounting Standards Codification

-Glossary Mortgage-Backed Securities

-URI http://asc.fasb.org/extlink&oid=6517959

Name:	us-gaap_MortgageBackedSecuritiesAvailableForSaleFairValueDisclosure
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant