Income Taxes - Summary of Income Tax Expense (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]
Estimated impact of the Tax Act									$ 15,441	$ 0	$ 0
Current expense (benefit):
Federal									8,483	25,176	7,978
State									5,500	12,904	(493)
Total current expense									13,983	38,080	7,485
Deferred expense (benefit):
Federal									49,169	10,658	(7,539)
State									27,679	16,496	23,992
Total deferred expense									76,848	27,154	16,453
Income tax expense	$ 34,958	$ 17,088	$ 20,714	$ 18,071	$ 18,336	$ 17,049	$ 15,460	$ 14,389	$ 90,831	$ 65,234	$ 23,938