Summary of Significant Accounting Policies - Deferred Costs & Impairment (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
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	3,900	4,100
Internal sales commissions	16,503		17,402
Third party commissions	11,051		11,802
Other	765		775
Total	28,319		$ 29,979
Recoverability of Long-Lived Assets
Impairment of real estate	$ 0	$ 0