Variable Interest Entities and Asset Securitizations - Additional Information (Detail) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015	Dec. 31, 2015
Variable Interest Entity Disclosure [Abstract]
Carrying value of loans in the Securitization trusts					$ 81,000,000
Combined outstanding principal amount of mortgage-backed securities issued by qualified special purpose trust held by unrelated parties					13,000,000
Other assets for its "investment" in the common securities recognized by the company of various trusts	$ 24,000,000		$ 24,000,000		24,000,000
Total assets of partnerships in which the company invested	1,100,000,000		1,100,000,000		1,100,000,000
Maximum exposure to loss of investments in real estate partnerships	287,000,000		287,000,000		295,000,000
Unfunded commitments included in company's maximum exposure to loss of investments in real estate partnerships	82,000,000		82,000,000		$ 78,000,000
Investments amortized to income tax expense	13,000,000	$ 10,000,000	34,000,000	$ 31,000,000
Federal tax credits and other federal tax benefits recognized	$ 18,000,000	$ 15,000,000	$ 46,000,000	$ 44,000,000