Income Taxes - Additional Information (Detail) - USD ($) $ in Thousands	3 Months Ended			6 Months Ended		12 Months Ended
	Jun. 30, 2018	Dec. 31, 2017	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Income Tax Disclosure [Abstract]
Accrued interest related to uncertain tax positions	$ 721		$ 609	$ 721	$ 609
Federal corporate tax rate				21.00%		35.00%
Deferred tax expense due to the Tax Act		$ 37,732
Effective tax rate	22.50%		34.25%	22.49%	34.25%