Income Tax - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Tax Disclosure [Abstract]
Tax Cuts and Jobs Act of 2017, change in tax rate, income tax benefit	$ 21,625,000	$ 0	$ 0
Deferred tax assets, valuation allowance	0
Unrecognized tax benefits	0	0
Income taxes, net of refunds	$ 36,000,000	$ 30,200,000	$ 27,300,000