Summary of Significant Accounting Policies - Prior Period Adjustment (Details) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Net cash provided by operating activities	$ 27,197	$ 31,666
Net cash used for investing activities	(183,558)	(64,320)
Net cash provided by (used in) financing activities	$ 154,093	30,377
Activities as originally presented
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Net cash provided by (used in) financing activities		30,666
Adjustment for shares tendered for tax withholding
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Net cash provided by operating activities		289
Net cash provided by (used in) financing activities		(289)
Accounting Standards Update 2016-15
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Net cash provided by operating activities		31,666
Net cash used for investing activities		(64,320)
Accounting Standards Update 2016-15 | Activities as originally presented
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Net cash provided by operating activities		36,598
Net cash used for investing activities		(69,485)
Accounting Standards Update 2016-15 | Adjustment relating to restricted cash
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Net cash provided by operating activities		(259)
Net cash used for investing activities		203
Accounting Standards Update 2016-15 | Adjustment relating to interest income presentation
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Net cash provided by operating activities		(4,962)
Net cash used for investing activities		$ 4,962