Summary of Significant Accounting Policies - Accumulated Other Comprehensive Loss (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018	Jun. 30, 2018
Accumulated Other Comprehensive Income (Loss) [Line Items]
Accumulated other comprehensive loss	$ (11,084)	$ (10,997)	$ (8,855)
AOCI Attributable to Parent
Accumulated Other Comprehensive Income (Loss) [Line Items]
Accumulated other comprehensive loss	(11,084)	(10,997)	(8,855)
Foreign currency translation adjustments
Accumulated Other Comprehensive Income (Loss) [Line Items]
Accumulated other comprehensive loss	(11,000)	(12,422)	(10,382)
Unrealized gains (losses) on interest rate swaps, net of tax
Accumulated Other Comprehensive Income (Loss) [Line Items]
Accumulated other comprehensive loss	$ (84)	$ 1,425	$ 1,527