Variable Interest Entities - Narrative (Detail) - USD ($)	3 Months Ended	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Variable Interest Entity [Line Items]
Investments in LIHTC operating partnerships	$ 230,000,000	$ 230,000,000	$ 178,000,000
Total Liabilities	4,000,000	4,000,000	4,000,000
Other expense		563,000,000	505,000,000	$ 344,000,000
LIHTC Investments [Member]
Variable Interest Entity [Line Items]
Investments in LIHTC operating partnerships	6,003,000,000	6,003,000,000	4,814,000,000
Total Liabilities	2,943,000,000	2,943,000,000	2,003,000,000
LIHTC Investments [Member] | Qualified affordable housing projects investment [Member]
Variable Interest Entity [Line Items]
Total Liabilities	476,000,000	476,000,000	462,000,000
Accrued Income And Other Assets [Member] | LIHTC Investments [Member]
Variable Interest Entity [Line Items]
Investments in LIHTC operating partnerships	1,300,000,000	1,300,000,000	1,200,000,000
Investments [Member] | LIHTC Investments [Member]
Variable Interest Entity [Line Items]
Amortization of investment		172,000,000	131,000,000
Tax credit of investment		171,000,000	144,000,000
Impairment expense related to tax reform	12,000,000
Other expense	3,000,000
Investments [Member] | Guaranteed Funds [Member]
Variable Interest Entity [Line Items]
Assets that can only be used to settle entities' obligations	4,000,000	4,000,000	1,000,000
Variable interest entity, liabilities	0	0	0
Fair Value, Measurements, Recurring [Member]
Variable Interest Entity [Line Items]
Total other investments	144,000,000	144,000,000	191,000,000
Principal Indirect Investments [Member] | Fair Value, Measurements, Recurring [Member]
Variable Interest Entity [Line Items]
Total other investments	$ 124,000,000	$ 124,000,000	$ 158,000,000