Variable Interest Entities and Asset Securitizations - Additional Information (Detail) (USD $)	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014	Dec. 31, 2014
Variable Interest Entity Disclosure [Abstract]
Carrying value of loans in the Securitization trusts	$ 93,000,000		$ 98,000,000
Combined outstanding principal amount of mortgage-backed securities issued by qualified special purpose trust held by unrelated parties	14,000,000		15,000,000
Residential mortgage loans securitized with Ginnie Mae	13,000,000	29,000,000
Other assets for its "investment" in the common securities recognized by the company of various trusts	34,000,000		34,000,000
Total assets of real estate partnerships in which the company invested	1,200,000,000		1,200,000,000
Maximum exposure to loss of investments in real estate partnerships	303,000,000		243,000,000
Unfunded commitments included in company's maximum exposure to loss of investments in real estate partnerships	88,000,000		56,000,000
Investments amortized to income tax expense	10,000,000	12,000,000
Tax credits and other tax benefits recognized	$ 14,000,000	$ 17,000,000