Significant Accounting Policies - Additional Information (Details) - USD ($)	3 Months Ended		12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2018	Sep. 30, 2018	Dec. 31, 2017
Significant Accounting Policies [Line Items]
Cumulative-effect adjustment to opening retained earnings	$ 400,000				$ 400,000
U.S. income tax rate	24.50%	35.00%			35.00%
Valuation allowance against net deferred tax assets	$ 0
Unrecorded excess tax benefits in additional paid-in-capital account	0
Restricted cash	$ 0				$ 0
Scenario, Forecast
Significant Accounting Policies [Line Items]
U.S. income tax rate			21.00%	24.50%