Income Taxes - Reconciliation of Beginning and Ending Balance of Unrecognized Tax Benefits (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Balance at January 1	$ 5,180	$ 4,071	$ 3,777
Additions based on tax positions related to the current year	1,145	1,054	861
Additions based on tax positions related to prior years	0	55	62
Reductions based on settlements of tax positions related to prior years	(439)	0	(624)
2017 Tax Act federal tax rate change	(1,859)	0	0
Other	(220)	0	(5)
Balance at December 31	$ 3,807	$ 5,180	$ 4,071