Derivative Financial Instruments - Additional Information (Detail) - USD ($)	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Derivative [Line Items]
Notional amount	$ 708,500,000				$ 660,000,000				$ 708,500,000	$ 660,000,000
Reclassified interest income (expense)	118,556,000	$ 108,478,000	$ 104,333,000	$ 105,801,000	109,917,000	$ 107,803,000	$ 99,340,000	$ 85,643,000	437,168,000	402,703,000	$ 366,116,000
Increase (decrease) in derivative assets									3,600,000
Increase (decrease) in derivative liabilities									2,700,000
Reclassifications out of Accumulated Other Comprehensive Income (Loss) [Member] | Interest Income [Member]
Derivative [Line Items]
Reclassified interest income (expense)									700,000
Reclassifications out of Accumulated Other Comprehensive Income (Loss) [Member] | Interest Expense [Member]
Derivative [Line Items]
Reclassified interest income (expense)									2,800,000
Fixed Interest Swap [Member]
Derivative [Line Items]
Notional amount	33,500,000				35,000,000				33,500,000	35,000,000
Variable Interest Rate Swap [Member]
Derivative [Line Items]
Notional amount	675,000,000				625,000,000				675,000,000	625,000,000
Interest Rate Lock Commitments [Member]
Derivative [Line Items]
Notional amount	29,900,000				40,300,000				29,900,000	40,300,000
Forward Commitments [Member]
Derivative [Line Items]
Notional amount	41,200,000				86,100,000				41,200,000	86,100,000
Customer Derivative Instrument [Member]
Derivative [Line Items]
Notional amount	826,600,000				582,700,000				826,600,000	582,700,000
Offsetting Counter Party Derivative Instrument [Member]
Derivative [Line Items]
Notional amount	826,600,000				582,700,000				826,600,000	582,700,000
Offsetting Counter Party Derivative Instrument [Member] | Foreign Currency Contracts [Member]
Derivative [Line Items]
Notional amount	800,000								800,000
Foreign Currency Forward Contract [Member] | Foreign Currency Contracts [Member]
Derivative [Line Items]
Notional amount	$ 800,000				$ 0				$ 800,000	$ 0