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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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 tax exempt income, equity in earnings (loss) of an unconsolidated subsidiary, minority noncontrolling interest income (loss), tax holiday, disposition of a business, disposition of an asset, repatriation of foreign earnings, repatriation of foreign earnings jobs creation act of 2004, increase (decrease) in enacted tax rate, prior year income taxes, increase (decrease) in deferred tax asset valuation allowance, and other adjustments.

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Income Taxes - Reconciliation of Effective Tax Rate (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Income Tax Disclosure [Abstract]
Federal tax based on statutory rate									$ 49,070	$ 28,340	$ 36,712
Effect of tax-exempt income									(7,064)	(7,282)	(7,558)
Interest and other nondeductible expenses									2,642	2,007	1,847
State taxes, net of federal Benefit									2,531	3,237	4,938
Tax credits									(12,012)	(11,690)	(8,756)
Other									(417)	1,257	1,313
Total income tax expense	$ 9,939us-gaap_IncomeTaxExpenseBenefit	$ 11,897us-gaap_IncomeTaxExpenseBenefit	$ 4,750us-gaap_IncomeTaxExpenseBenefit	$ 8,163us-gaap_IncomeTaxExpenseBenefit	$ 9,175us-gaap_IncomeTaxExpenseBenefit	$ 7,357us-gaap_IncomeTaxExpenseBenefit	$ 4,213us-gaap_IncomeTaxExpenseBenefit	$ (4,876)us-gaap_IncomeTaxExpenseBenefit	$ 34,750us-gaap_IncomeTaxExpenseBenefit	$ 15,869us-gaap_IncomeTaxExpenseBenefit	$ 28,496us-gaap_IncomeTaxExpenseBenefit
Effective tax rate									24.80%	19.60%	27.20%