Income Taxes - Taxes Computed at U.S. Statutory Federal Income Tax Rate (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]
Taxes at the U.S. federal income tax rate									$ 26,745	$ 23,398	$ 9,124
U.S./Canadian tax rate differential									(94)	21	(18)
U.S. state taxes net of federal benefit									1,339	1,045	401
Non-deductible expenses									(104)	144	166
Domestic production activities deduction									(1,870)	(1,719)	(356)
Tax credits									0	(12)	(147)
Valuation allowance									(60)	27	272
Tax Cuts and Jobs Act of 2017									(9,168)	0	0
Other									(222)	(77)	(106)
Total income tax (expense) benefit	$ (1,208)	$ 5,674	$ 6,370	$ 5,730	$ 6,879	$ 3,014	$ 6,604	$ 6,330	$ 16,566	$ 22,827	$ 9,336
Statutory tax rate									35.00%	35.00%	35.00%
Effective rate (as a percent)									21.68%	34.15%	35.81%