Commitments and Contingent Liabilities - Additional Information (Detail) - USD ($)		12 Months Ended
	Apr. 21, 2017	Dec. 31, 2017	Dec. 31, 2016
Loss Contingencies [Line Items]
Loan commitments outstanding		$ 4,224,719,000	$ 2,823,396,000
Loan commitments expiry period		1 year
George Mason [Member]
Loss Contingencies [Line Items]
Payments to investors to repurchase loans previously sold		$ 59,000
Reserve for possible losses due to the repurchase of loans previously sold to investors	$ 575,000	516,000
Expense related to the reserve for possible losses due to the repurchase of loans previously sold to investors		0
Commitments to Extend Credit [Member] | George Mason [Member] | Short-term Contract with Customer [Member]
Loss Contingencies [Line Items]
Additional commitments to extend credit		229,630,000
Commercial Letters of Credit [Member]
Loss Contingencies [Line Items]
Letters of credit issued		0	9,000
Standby Letters of Credit [Member]
Loss Contingencies [Line Items]
Letters of credit issued		$ 147,017,000	$ 121,584,000