Guarantee Arrangements, Pledged Assets and Repurchase Agreements - Summary of Change in Estimated Repurchase Liability for Loans Sold to GSEs and Other Third Parties (Detail) - Obligation to Repurchase Receivables Sold [Member] - USD ($)	3 Months Ended			9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Mar. 31, 2014	Sep. 30, 2015	Sep. 30, 2014	Dec. 31, 2014
Loss Contingencies [Line Items]
Outstanding repurchase demands	$ 3,000,000			$ 3,000,000		$ 3,000,000
Estimated Repurchase Liability For Loans Sold [Roll Forward]
Balance at beginning of period	21,000,000	$ 34,000,000	$ 99,000,000	27,000,000	$ 99,000,000
Increase (decrease) in liability recorded through earnings	(2,000,000)	(4,000,000)		(7,000,000)	(38,000,000)
Realized losses	0	0		(1,000,000)	(31,000,000)
Balance at end of period	19,000,000	$ 30,000,000		19,000,000	$ 30,000,000
Range of possible losses in excess of recorded repurchase liability, minimum	0			0
Range of possible losses in excess of recorded repurchase liability, maximum	$ 25,000,000			$ 25,000,000
Federal Home Loan Mortgage Corporation [Member]
Estimated Repurchase Liability For Loans Sold [Roll Forward]
Realized losses			(25,000,000)
Release of repurchase reserves			$ 34,000,000