Basis of Presentation and Significant Accounting Policies - 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
Basis Of Presentation And Significant Accounting Policies [Line Items]
Loan placed on nonaccrual when past due, number of days									90 days
Days delinquent required for charge off									90 days
Accrued interest coverage by FDIC, number of days									90 days
Percentage of income tax examination likelihood of tax benefits									50.00%
Tax benefit recorded	$ 40,405,000	$ 11,459,000	$ 10,584,000	$ 10,491,000	$ 14,710,000	$ 10,969,000	$ 30,812,000	$ 9,671,000	$ 72,939,000	$ 66,162,000	$ 46,177,000
Retained earnings	413,130,000				$ 390,292,000				413,130,000	390,292,000
Unrealized holding gains on available-for-sale portfolio									3,455,000	(35,206,000)	(3,058,000)
Stock-based compensation expense									6,275,000	$ 7,318,000	$ 4,255,000
Accounting Standards Update 2016-09 [Member]
Basis Of Presentation And Significant Accounting Policies [Line Items]
Stock-based compensation expense									100,000
Accounting Standards Update 2017-12 [Member]
Basis Of Presentation And Significant Accounting Policies [Line Items]
Retained earnings	500,000								500,000
Reclassification of securities	324,000,000								324,000,000
Unrealized holding gains on available-for-sale portfolio									26,100,000
Accounting Standards Update 2017-12 [Member] | States and Political Subdivisions [Member]
Basis Of Presentation And Significant Accounting Policies [Line Items]
Reclassification of securities	447,000,000								447,000,000
Maximum [Member]
Basis Of Presentation And Significant Accounting Policies [Line Items]
Value of small commercial loans on nonaccrual status or 90 days or more delinquent									$ 250,000
Maturity of short-term securities sold under agreements to repurchase									4 days
Maximum [Member] | Accounting Standards Update 2016-02 [Member]
Basis Of Presentation And Significant Accounting Policies [Line Items]
Anticipated increase in assets and liabilities									$ 100,000,000
Maximum [Member] | Other Intangible Assets [Member]
Basis Of Presentation And Significant Accounting Policies [Line Items]
Estimated useful lives of core deposits and customer relationships									15 years
Minimum [Member]
Basis Of Presentation And Significant Accounting Policies [Line Items]
Maturity of short-term securities sold under agreements to repurchase									1 day
Minimum [Member] | Accounting Standards Update 2016-02 [Member]
Basis Of Presentation And Significant Accounting Policies [Line Items]
Anticipated increase in assets and liabilities									$ 80,000,000
Minimum [Member] | Other Intangible Assets [Member]
Basis Of Presentation And Significant Accounting Policies [Line Items]
Estimated useful lives of core deposits and customer relationships									5 years
Expenses up to $275.0 [Member]
Basis Of Presentation And Significant Accounting Policies [Line Items]
Expenses sharing reimbursement threshold									80.00%
Loss amount covered at 80% up to $275 million	$ 275,000,000								$ 275,000,000
Building and Building Improvements [Member] | Maximum [Member]
Basis Of Presentation And Significant Accounting Policies [Line Items]
Useful lives for premises and equipment, years									39 years
Building and Building Improvements [Member] | Minimum [Member]
Basis Of Presentation And Significant Accounting Policies [Line Items]
Useful lives for premises and equipment, years									15 years
Furniture and Equipment [Member] | Maximum [Member]
Basis Of Presentation And Significant Accounting Policies [Line Items]
Useful lives for premises and equipment, years									10 years
Furniture and Equipment [Member] | Minimum [Member]
Basis Of Presentation And Significant Accounting Policies [Line Items]
Useful lives for premises and equipment, years									3 years