The aggregate amount of recurring noncash expense charged against earnings in the period to allocate the cost of assets over their estimated remaining economic lives.

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Variable Interest Entities - Additional Information (Detail) - USD ($)	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Variable Interest Entity [Line Items]
Investments in operating partnerships	$ 220,000,000		$ 230,000,000
Liabilities related to investments in qualified affordable housing projects	1,000,000		4,000,000
Recurring [Member]
Variable Interest Entity [Line Items]
Alternative Investment	122,000,000		144,000,000
Recurring [Member] \| Principal Investments Indirect [Member]
Variable Interest Entity [Line Items]
Alternative Investment	101,000,000		124,000,000
LIHTC Investments [Member]
Variable Interest Entity [Line Items]
Investments in operating partnerships	5,955,000,000		6,003,000,000
Liabilities related to investments in qualified affordable housing projects	2,589,000,000		2,943,000,000
Tax credit of investment	115,000,000
LIHTC Investments [Member] \| Qualified affordable housing projects investment [Member]
Variable Interest Entity [Line Items]
Liabilities related to investments in qualified affordable housing projects	447,000,000		476,000,000
LIHTC Investments [Member] \| Accrued Income And Other Assets [Member]
Variable Interest Entity [Line Items]
Investments in operating partnerships	1,300,000,000
LIHTC Investments [Member] \| Investments [Member]
Variable Interest Entity [Line Items]
Amortization of investment	126,000,000	$ 124,000,000
Tax credit of investment	122,000,000
Guaranteed Funds [Member] \| Investments [Member]
Variable Interest Entity [Line Items]
VIE, assets that can only be used to settle obligations	0		$ 4,000,000
VIE, liabilities	$ 0