Variable Interest Entities and Asset Securitizations - Additional Information (Detail) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014	Dec. 31, 2014
Variable Interest Entity Disclosure [Abstract]
Residential mortgage loans securitized with Ginnie Mae	$ 15,000,000	$ 35,000,000	$ 51,000,000	$ 110,000,000
Carrying value of loans in the Securitization trusts	84,000,000		84,000,000		$ 98,000,000
Combined outstanding principal amount of mortgage-backed securities issued by qualified special purpose trust held by unrelated parties	13,000,000		13,000,000		15,000,000
Other assets for its "investment" in the common securities recognized by the company of various trusts	24,000,000		24,000,000		34,000,000
Total assets of real estate partnerships in which the company invested	1,200,000,000		1,200,000,000		1,200,000,000
Maximum exposure to loss of investments in real estate partnerships	301,000,000		301,000,000		243,000,000
Unfunded commitments included in company's maximum exposure to loss of investments in real estate partnerships	85,000,000		85,000,000		$ 56,000,000
Investments amortized to income tax expense	10,000,000	14,000,000	31,000,000	39,000,000
Tax credits and other tax benefits recognized	$ 15,000,000	$ 18,000,000	$ 44,000,000	$ 53,000,000