Income Taxes - Reconciliation of Tax Expense at the Federal Statutory Tax Rate (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Income Tax Disclosure [Abstract]
Tax expense at the federal statutory tax rate	$ 31,910	$ 28,295	$ 27,651
Increase (decrease) in tax expense from:
Tax-exempt income, net	(571)	(621)	(783)
Modified endowment life contracts	(755)	(812)	(870)
State tax expense, net of federal tax benefit	1,469	2,303	2,374
Federal tax credits	(6,479)	(4,132)	(2,455)
Other, net	1,710	1,492	1,187
Total income taxes	$ 27,284us-gaap_IncomeTaxExpenseBenefit	$ 26,525us-gaap_IncomeTaxExpenseBenefit	$ 27,104us-gaap_IncomeTaxExpenseBenefit

Income tax reconciliation modified endowment life contracts tax benefit.

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

+ References

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-Publisher FASB

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The amount of income tax expense or benefit for the period computed by applying the domestic federal statutory tax rates to pretax income from 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 other adjustments.

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