Income Taxes - Summary of Income Tax Expense Allocated Based on Separate Tax Return Basis (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Income Tax Disclosure [Abstract]
Current income taxes: Federal taxes	$ 18,111us-gaap_CurrentFederalTaxExpenseBenefit	$ 33,614us-gaap_CurrentFederalTaxExpenseBenefit	$ 22,182us-gaap_CurrentFederalTaxExpenseBenefit
Current income taxes: State taxes	396	(116)	416
Current income taxes	18,507us-gaap_CurrentIncomeTaxExpenseBenefit	33,498us-gaap_CurrentIncomeTaxExpenseBenefit	22,598us-gaap_CurrentIncomeTaxExpenseBenefit
Deferred income taxes: Federal taxes	44,113	11,258	21,175
Deferred income taxes: State taxes	0	0	0
Deferred income taxes	44,113us-gaap_DeferredIncomeTaxExpenseBenefit	11,258us-gaap_DeferredIncomeTaxExpenseBenefit	21,175us-gaap_DeferredIncomeTaxExpenseBenefit
Income taxes	$ 62,620us-gaap_IncomeTaxExpenseBenefit	$ 44,756us-gaap_IncomeTaxExpenseBenefit	$ 43,773us-gaap_IncomeTaxExpenseBenefit

Amount of current federal tax expense (benefit) pertaining to income (loss) from continuing operations.

Amount of current income tax expense (benefit) pertaining to taxable income (loss) from continuing operations.

Amount of current state and local tax expense (benefit) pertaining to income (loss) from continuing operations.

Amount of deferred federal income tax expense (benefit) pertaining to income (loss) from continuing operations.

Amount of deferred income tax expense (benefit) pertaining to income (loss) from continuing operations.

Amount of deferred state and local tax expense (benefit) pertaining to income (loss) from continuing operations.