Income Taxes - Provision for Income Taxes (Detail) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Tax Disclosure [Abstract]
Federal income taxes currently payable									$ 0	$ 23,735	$ 17,385
State income taxes currently payable									0	2,242	769
Federal income taxes deferred									31,915	35,955	24,664
Estimated revaluation of deferred tax assets	$ 39,300								39,300
State income taxes deferred									1,724	4,230	3,359
Deferred income tax expense									72,939	40,185	28,023
Income tax expense	$ 40,405	$ 11,459	$ 10,584	$ 10,491	$ 14,710	$ 10,969	$ 30,812	$ 9,671	$ 72,939	$ 66,162	$ 46,177