Summary of Significant Accounting Policies - Deferred Costs & Impairment (Details) - USD ($)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Summary of Significant Accounting Policies
Impairment loss related to intangible assets	$ 0	$ 0	$ 0
Goodwill	40,646,000	40,646,000
Impairment loss related to goodwill	0	0	0
Deferred Costs
Amortization of initial direct costs	14,200,000	15,600,000	16,300,000
Internal sales commissions	15,064,000	14,199,000
Third party commissions	10,845,000	9,855,000
Other	462,000	597,000
Total	26,371,000	24,651,000
Recoverability of Long-Lived Assets
Impairment of long-lived assets	$ 0	$ 0	$ 0