Variable Interest Entities - Additional Information (Detail) - USD ($)	Dec. 31, 2017	Dec. 31, 2016
Variable Interest Entity Disclosure [Abstract]
Other assets for its "investment" in the common securities recognized by the company of various trusts	$ 23,000,000	$ 24,000,000
Total assets of partnerships in which the company invested	1,000,000,000	1,000,000,000
Maximum exposure to loss of investments in real estate partnerships	420,000,000	294,000,000
Unfunded commitments included in company's maximum exposure to loss of investments in real estate partnerships	$ 201,000,000	$ 102,000,000