Summary of Significant Accounting Policies - Deferred Costs & Impairment (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018
Summary of Significant Accounting Policies
Impairment loss related to intangible assets	$ 0	$ 0
Goodwill	40,646		$ 40,646
Impairment loss related to goodwill	0	0
Deferred Costs
Amortization of initial direct costs	3,400	3,900
Internal sales commissions	13,732		14,199
Third party commissions	9,594		9,855
Other	549		597
Total	23,875		$ 24,651
Recoverability of Long-Lived Assets
Impairment of long-lived assets	$ 0	$ 0