Residential Investment Securities - Narrative (Detail) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
Mar. 31, 2016	Mar. 31, 2015
Mortgage-Backed Securities Sold:
Residential Investment securities sold, carrying value	$ 3,500.0	$ 14,900.0
Residential Investment securities sold, realized gain	$ (1.7)	$ 62.3

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

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