Income Taxes - Gross Unrecognized Tax Benefits Reconciliation (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Reconciliation of the beginning and ending amount of gross unrecognized tax benefits
Balance as of beginning of period	$ 13,604	$ 12,597	$ 13,528
Additions for current year tax positions	3,243	3,274	3,371
Additions for tax positions of prior years	342	141	949
Reductions for tax positions of prior years	(114)	(173)	(1,807)
Reductions due to lapses of applicable statutes of limitations	(2,693)	(2,235)	(3,071)
Reductions due to Tax Act	(509)
Reductions due to settlements with taxing authorities	(293)		(373)
Balance as of end of period	13,580	13,604	$ 12,597
Unrecognized tax benefits realized that would have reduced income tax expense	$ 10,700	$ 9,100