Derivative Financial Instruments - Additional Information (Detail) - USD ($)	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2017	Dec. 31, 2016
Derivative [Line Items]
Notional amount	$ 760,000,000		$ 760,000,000			$ 660,000,000
Reclassified interest income (expense)	108,478,000	$ 107,803,000	318,612,000	$ 292,786,000
Increase (decrease) in derivative assets			2,500,000
Increase (decrease) in derivative liabilities			6,300,000
Scenario, Forecast [Member] | Reclassifications out of Accumulated Other Comprehensive Income (Loss) [Member] | Interest Income [Member]
Derivative [Line Items]
Reclassified interest income (expense)					$ 200,000
Scenario, Forecast [Member] | Reclassifications out of Accumulated Other Comprehensive Income (Loss) [Member] | Interest Expense [Member]
Derivative [Line Items]
Reclassified interest income (expense)					$ 3,900,000
Fixed Interest Swap [Member]
Derivative [Line Items]
Notional amount	35,000,000		35,000,000			35,000,000
Variable Interest Rate Swap [Member]
Derivative [Line Items]
Notional amount	725,000,000		725,000,000			625,000,000
Interest Rate Lock Commitments [Member]
Derivative [Line Items]
Notional amount	57,900,000		57,900,000			40,300,000
Forward Commitments [Member]
Derivative [Line Items]
Notional amount	75,500,000		75,500,000			86,100,000
Customer Derivative Instrument [Member]
Derivative [Line Items]
Notional amount	801,200,000		801,200,000			582,700,000
Offsetting Counter Party Derivative Instrument [Member]
Derivative [Line Items]
Notional amount	801,200,000		801,200,000			$ 582,700,000
Offsetting Counter Party Derivative Instrument [Member] | Foreign Currency Contracts [Member]
Derivative [Line Items]
Notional amount	700,000		700,000
Foreign Currency Forward Contract [Member] | Foreign Currency Contracts [Member]
Derivative [Line Items]
Notional amount	$ 700,000		$ 700,000