Variable Interest Entities - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Variable Interest Entity Disclosure [Abstract]
Loss on securitization of assets	$ 0	$ 0	$ 0
Other assets for its "investment" in the common securities recognized by the company of various trusts	23,000,000	23,000,000
Total assets of partnerships in which the company invested	1,100,000,000	1,000,000,000
Maximum exposure to loss of investments in real estate partnerships	523,000,000	420,000,000
Unfunded commitments included in company's maximum exposure to loss of investments in real estate partnerships	$ 280,000,000	$ 201,000,000