Fair Value Measurements - Quantitative Information about Nonrecurring Fair Value Measurement of Assets and Liabilities Classified as Level 3 (Detail) - Level 3 [Member] - USD ($) $ in Millions		9 Months Ended	12 Months Ended
		Sep. 30, 2017	Dec. 31, 2016
Fair Value, Assets and Liabilities Measured on a Nonrecurring Basis [Line Items]
Assets, Fair Value Disclosure, Nonrecurring		$ 336	$ 1,047
Residential mortgages [Member]
Fair Value, Assets and Liabilities Measured on a Nonrecurring Basis [Line Items]
Assets, Fair Value Disclosure, Nonrecurring	[1]	3	769
Impaired commercial loans [Member]
Fair Value, Assets and Liabilities Measured on a Nonrecurring Basis [Line Items]
Assets, Fair Value Disclosure, Nonrecurring	[2]	$ 333	$ 278
Fair Value, Measurements, Nonrecurring [Member] | Residential mortgages [Member] | Minimum [Member] | Third Party Appraisal [Member]
Fair Value, Assets and Liabilities Measured on a Nonrecurring Basis [Line Items]
Fair Value Inputs, Loss Severity		6.00%	0.00%
Fair Value Inputs, Discount Rate			8.00%
Fair Value, Measurements, Nonrecurring [Member] | Residential mortgages [Member] | Maximum [Member] | Third Party Appraisal [Member]
Fair Value, Assets and Liabilities Measured on a Nonrecurring Basis [Line Items]
Fair Value Inputs, Loss Severity		100.00%	100.00%
Fair Value Inputs, Discount Rate			14.00%
Fair Value, Measurements, Nonrecurring [Member] | Impaired commercial loans [Member] | Minimum [Member] | Third Party Appraisal [Member]
Fair Value, Assets and Liabilities Measured on a Nonrecurring Basis [Line Items]
Fair Value Inputs, Loss Severity		3.00%	4.00%
Fair Value, Measurements, Nonrecurring [Member] | Impaired commercial loans [Member] | Maximum [Member] | Third Party Appraisal [Member]
Fair Value, Assets and Liabilities Measured on a Nonrecurring Basis [Line Items]
Fair Value Inputs, Loss Severity		100.00%	100.00%

[1] At September 30, 2017 and December 31, 2016, the fair value of the loans held for sale was below cost. Certain residential mortgage loans held for sale have been classified as Level 3 fair value measurements within the fair value hierarchy, including certain residential mortgage loans held for sale for which the underlying real estate properties used to determine fair value are illiquid assets as a result of market conditions and, at December 31, 2016, certain residential mortgage loans which were transferred to held for sale during 2016 and substantially sold during the nine months ended September 30, 2017 for which significant inputs in estimating fair value were unobservable. Additionally, the fair value of these properties is affected by, among other things, the location, the payment history and the completeness of the loan documentation.

[2] Certain commercial loans have undergone troubled debt restructurings and are considered impaired. As a matter of practical expedient, we measure the credit impairment of a collateral-dependent loan based on the fair value of the collateral asset. The collateral often involves real estate properties that are illiquid due to market conditions. As a result, these loans are classified as a Level 3 fair value measurement within the fair value hierarchy.