Contingencies - Additional Information (Detail) $ in Millions	12 Months Ended
	Dec. 31, 2017 USD ($)
Document Period End Date	Dec. 31, 2017
Minimum [Member]
Aggregate range of reasonably possible losses on legal matters in excess of the accrued liability	$ 0
Maximum [Member]
Aggregate range of reasonably possible losses on legal matters in excess of the accrued liability	160
Cash payment for litigation [Member] | Potential Mortgage Servicing Settlement and Claims [Member]
Loss Contingency, Damages Awarded, Value	50
Consumer relief obligation [Member] | Potential Mortgage Servicing Settlement and Claims [Member]
Loss Contingency, Damages Awarded, Value	$ 500