Variable Interest Entities and Asset Securitizations - Additional Information (Detail) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019
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	138,626,743,000		138,626,743,000		119,872,757,000
Investment in partnership carrying amount	791,000,000		791,000,000		748,000,000
Unfunded commitments includes carrrying amount of its investments	380,000,000		380,000,000		414,000,000
Maximum exposure to loss of investments in real estate partnerships	978,000,000		978,000,000
Investments amortized to income tax expense	22,000,000	$ 17,000,000	65,000,000	51,000,000
Federal tax credits and other federal tax benefits recognized	26,000,000	$ 21,000,000	78,000,000	$ 62,000,000
Variable Interest Entity Primary Beneficiary [Member]
Total assets of partnerships in which the company invested	$ 2,100,000,000		$ 2,100,000,000		$ 1,500,000,000