Income Taxes - Rollforward 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]
Beginning balance		$ 2,039	$ 1,408	$ 912
Additions for tax positions relating to the current year		845	640	480
Additions for tax positions relating to prior years		13,389	0	37
Reductions for tax positions relating to prior years		(25)	(9)	(21)
Ending balance		16,248	2,039	1,408
Interest and penalties not included above:
Interest expense, net of tax effect, and penalties	[1]	118	49	20
Accrued interest and penalties, net of tax effect, at end of year	[1]	$ 191	$ 73	$ 24

[1] Included in income tax expense in the Consolidated Statements of Income.