Variable Interest Entities and Asset Securitizations - Additional Information (Detail) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Variable Interest Entity Disclosure [Abstract]
Loss on securitization of assets			$ 0	$ 0
Other assets for its "investment" in the common securities recognized by the company of various trusts	$ 23,000,000		23,000,000		$ 23,000,000
Total assets of partnerships in which the company invested	1,000,000,000		1,000,000,000		1,000,000,000
Maximum exposure to loss of investments in real estate partnerships	457,000,000		457,000,000		420,000,000
Unfunded commitments included in company's maximum exposure to loss of investments in real estate partnerships	215,000,000		215,000,000		$ 201,000,000
Investments amortized to income tax expense	13,000,000	$ 11,000,000	39,000,000	35,000,000
Federal tax credits and other federal tax benefits recognized	$ 15,000,000	$ 16,000,000	$ 47,000,000	$ 47,000,000