Variable Interest Entities and Asset Securitizations - Additional Information (Detail) - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Variable Interest Entity Disclosure [Abstract]
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	461,000,000		461,000,000		420,000,000
Unfunded commitments included in company's maximum exposure to loss of investments in real estate partnerships	229,000,000		229,000,000		$ 201,000,000
Investments amortized to income tax expense	13,000,000	$ 11,000,000	26,000,000	$ 24,000,000
Federal tax credits and other federal tax benefits recognized	$ 16,000,000	$ 15,000,000	$ 32,000,000	$ 31,000,000