Income Taxes - Summary of Differences in Taxes from Continuing Operations Computed at Statutory Rate (Detail) - USD ($) $ in Thousands	3 Months Ended
Mar. 31, 2019	Mar. 31, 2018
Income Tax Disclosure [Abstract]
Provision at statutory rate	$ 14,570	$ 11,117
Tax-exempt interest	(2,531)	(2,191)
Section 291/265 interest disallowance	111	62
Company-owned life insurance income	(669)	(547)
Tax-exempt income	(3,089)	(2,676)
State income taxes	1,999	1,188
Interim period effective rate adjustment	688	92
Tax credit investments - federal	(420)	(5,769)
Other, net	(644)	1,005
Income tax expense	$ 13,104	$ 4,957
Effective tax rate	18.90%	9.40%

Income tax reconciliation, non-deductible expense, interest disallowance.

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Income tax reconciliation tax exempt interest.

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Percentage of current income tax expense (benefit) and deferred income tax expense (benefit) pertaining to continuing operations.

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Amount of current income tax expense (benefit) and deferred income tax expense (benefit) pertaining to continuing operations.

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Amount of the difference between reported income tax expense (benefit) and expected income tax expense (benefit) computed by applying the domestic federal statutory income tax rates to pretax income (loss) from continuing operations, attributable to increase (decrease) in the income tax rates.

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Amount of the difference between reported income tax expense (benefit) and expected income tax expense (benefit) computed by applying the domestic federal statutory income tax rates to pretax income (loss) from continuing operations attributable to income (loss) exempt from income taxes.

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