Significant Accounting Policies Significant Accounting Policies - Revenue Recognition (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Net service revenue	$ 502,024	$ 257,535	$ 793,078	$ 501,784
Provision for bad debts		$ 0		$ 0
Allowance for uncollectible accounts					$ 0
Accounting Standards Update 2014-09
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Net service revenue	$ 7,700		$ 12,600