Summary of Significant Accounting Policies (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Jan. 01, 2016
Recent Accounting Pronouncements
Estimated increase in sales and marketing expense	$ 4,643	$ 4,465	$ 13,560	$ 13,187
ASU 2016-02
Recent Accounting Pronouncements
Estimated increase in straight-line rental expense							$ 4,000
ASU 2016-02 | Minimum
Recent Accounting Pronouncements
Right of use asset								$ 100,000
Right of use liability								100,000
ASU 2016-02 | Maximum
Recent Accounting Pronouncements
Right of use asset								300,000
Right of use liability								$ 300,000
Forecast | ASU 2016-02
Recent Accounting Pronouncements
Estimated increase in straight-line rental expense					$ 4,000	$ 4,000
Estimated increase in sales and marketing expense					$ 1,500