Income Taxes - Unrecognized Tax Benefits (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Tax Disclosure [Abstract]
Total provision for UTP as of January 1,	$ 7,660	$ 7,278	$ 5,557
Current year acquisitions	0	911	0
Increases related to current year tax positions	62	0	347
Changes in tax positions of a prior period	55	0	1,934
Lapse due to statute of limitations	0	(342)	0
Settlements with taxing authorities	0	(187)	(560)
Total provision for UTP as of December 31,	7,777	7,660	7,278
Deferred tax assets available to offset provision for UTP	(7,360)	(7,279)	(6,936)
Total provision for UTP as of December 31,	$ 417	$ 381	$ 342