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Income Taxes - Schedule of Effective Income Tax Rate Reconciliation (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Tax Expense (Benefit), Continuing Operations, Income Tax Reconciliation [Abstract]
Tax at statutory rate, Amount	$ 36,304	$ 29,830	$ 24,843
Income Tax Reconciliation, Bank Owned Life Insurance Income	(1,995)	(1,883)	(1,894)
Tax-exempt interest income, net, Amount	(2,709)	(2,434)	(2,232)
Tax credits, Amount	(11)	0	(61)
Other, Amount	99	182	136
Total tax provision (benefit), Amount	$ 48,561	$ 25,639	$ 20,836
Effective Income Tax Rate, Continuing Operations, Tax Rate Reconciliation [Abstract]
Tax at statutory rate, Pretax Income	35.00%	35.00%	35.00%
Income from bank owned life insurance, Pretax Income	(2.00%)	(2.00%)	(3.00%)
Effective Income Tax Rate Reconciliation, State and Local Income Taxes, Amount	$ 164	$ (56)	$ 44
Effective Income Tax Rate Reconciliation, State and Local Income Taxes, Percent	0.00%	0.00%	0.00%
Tax-exempt interest income, net, Pretax Income	(3.00%)	(3.00%)	(3.00%)
Tax credits, Pretax Income	(0.00%)	(0.00%)	(0.00%)
Effective Income Tax Rate Reconciliation, Change in Enacted Tax Rate, Amount	$ 16,709	$ 0	$ 0
Effective Income Tax Rate Reconciliation, Deduction, Percent	(17.00%)	0.00%	0.00%
Other, Pretax Income	0.00%	0.00%	0.00%
Total tax provision (benefit), Pretax Income	47.00%	30.00%	29.00%

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