Fair Value Measurements - Additional Information (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Weighted average collateral loss severity rate (percent)	45.00%		45.00%		51.00%
Minimum period to update carrying values for real estate owned properties			45 days
Real Estate Secured Receivables Held for Sale [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets transferred from Level 3 to Level 2	$ 1,048	$ 757	$ 5,267	$ 5,683