Commitments and Contingent Liabilities - Additional Information (Detail) - USD ($)	6 Months Ended
	Jun. 30, 2018	Dec. 31, 2017
Loss Contingencies [Line Items]
Loan commitments outstanding	$ 3,899,439,000	$ 4,224,719,000
Loan commitments expiry period	1 year
George Mason [Member]
Loss Contingencies [Line Items]
Reserve for possible losses due to the repurchase of loans previously sold to investors	$ 516,000
Commitments to Extend Credit [Member] | George Mason [Member] | Short-term Contract with Customer [Member]
Loss Contingencies [Line Items]
Additional commitments to extend credit	324,487,000
Commercial Letters of Credit [Member]
Loss Contingencies [Line Items]
Letters of credit issued	0	0
Standby Letters of Credit [Member]
Loss Contingencies [Line Items]
Letters of credit issued	$ 145,778,000	$ 147,017,000