Income Taxes - Additional Information (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Income Tax Disclosure [Abstract]
Estimated federal taxable income	$ 388,000	$ 362,978
Statutory federal income tax rate	35.00%
Effective tax rate	37.90%	36.36%	34.35%
Income tax expense due to reversal of previously recognized tax deductions	7,000
Income tax expense reduction			9,154
Unrecognized tax benefits that would affect effective tax rate	29,902
Accrued interest on unrecognized tax benefits		625
Reversal of accrued interest on unrecognized tax benefits	184		4,116
Total accrued interest on unrecognized tax benefits	21,096	21,281
Reduction in unrecognized tax benefits	$ 6,339