Commitments and Contingencies - Additional Information (Detail) - USD ($)	3 Months Ended	12 Months Ended
	Mar. 31, 2018	Dec. 31, 2018	Dec. 31, 2017
Loss Contingencies [Line Items]
Commitments to extend credit to commercial customers		$ 8,600,000,000	$ 8,100,000,000
Maximum credit risk for recourse associated with loans sold under Federal National Mortgage Association Delegated Underwriting and Servicing program		$ 3,400,000,000	$ 3,300,000,000
Period of noncancelable operating lease in years		23 years
Increase in litigation reserve	$ 135,000,000
Payment of legal settlement expense	$ 200,000,000
Minimum [Member]
Loss Contingencies [Line Items]
Range of reasonably possible losses		$ 0
Maximum [Member]
Loss Contingencies [Line Items]
Range of reasonably possible losses		$ 50,000,000