Income Tax Provision - Additional Information (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Income Tax Disclosure [Abstract]
Effective income tax rates	4.10%	10.60%
Federal income tax rate	21.00%	35.00%
Excess tax benefit	$ 9,100	$ 10,100
Effective income tax rate excluding excess tax benefit	27.20%	38.20%
Change in tax rate, income tax benefit	$ 26,000
Income tax payments	$ 218	$ 161