Summary of Significant Accounting Policies - Revenue - Contract Fulfillment Assets (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Jan. 01, 2018
Commissions asset
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Net Carrying Amount	$ 22,491
Accumulated Amortization	(31,386)	$ (21,173)
Gross Carrying Amount	53,877	42,072
Intake Costs asset
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Net Carrying Amount	15,420
Accumulated Amortization	(22,185)	(14,954)
Gross Carrying Amount	$ 37,605	31,604
Accounting Standards Update 2014-09 | Commissions asset
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Net Carrying Amount		20,899
Accounting Standards Update 2014-09 | Intake Costs asset
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Net Carrying Amount		$ 16,650