Income Taxes - Additional Information (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended	9 Months Ended		12 Months Ended	48 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Jun. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017	Dec. 31, 2016
Income Tax [Abstract]
Deferred Tax Assets, Net	$ 54,100			$ 54,100		$ 31,700
Valuation allowance	0			$ 0
U.S. federal statutory income tax rate, percent				21.00%	35.00%
Unrecognized tax benefits	0			$ 0
Deferred tax adjustment related to reduction in U.S. federal statutory income tax rate.						$ 4,000
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Current income tax expense (benefit)	$ (1,152)	$ 15,224		$ 955	$ 44,636
Error Correction [Member]
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Current income tax expense (benefit)			$ 756		$ 3,662		$ 2,906