Significant Accounting Policies - Prior Period Adjustments (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Net cash provided by operating activities	$ 58,018	$ 59,989	$ 33,266
Net cash used in investing activities	$ (102,426)	(226,253)	(57,961)
Accounting Standards Update 2016-15
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Net cash provided by operating activities		59,989	33,266
Net cash used in investing activities		(226,253)	(57,961)
Accounting Standards Update 2016-15 | Operating activities as originally presented
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Net cash provided by operating activities		59,770	33,086
Net cash used in investing activities		(226,461)	(56,381)
Accounting Standards Update 2016-15 | Adjustments
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Net cash provided by operating activities		219	180
Net cash used in investing activities		$ 208	$ (1,580)