Summary of Significant Accounting Policies - Revenue Recognition, Unbilled Receivables, and Estimated Liability for Refunds and Appeals (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Summary of Significant Accounting Policies
Estimated liability for refunds and appeals	$ 57,971	$ 62,539
Estimated allowance for refunds and appeals	30,996	41,020
Accounts receivable
Summary of Significant Accounting Policies
Unbilled receivables	54,138	51,643
Unbilled receivables arising from deferred billing	$ 5,457	$ 6,137