Revenue Recognition - Balance Sheet Impact (Details) - USD ($) $ in Thousands	Dec. 31, 2018		Dec. 31, 2017		Dec. 31, 2016	Dec. 31, 2015
Revenue
Prepaid expenses and other current assets	$ 34,000		$ 30,683
Accrued income taxes	5,062		11,503
Deferred revenue and customer deposits	9,845		9,420		$ 7,078
Retained earnings	$ 646,214	[1]	668,587	[1]	644,622	$ 573,286
As previously reported
Revenue
Prepaid expenses and other current assets			30,800
Accrued income taxes			11,391
Deferred revenue and customer deposits			9,969
Retained earnings			668,267		643,836	566,622
Adjustment [Member] | Accounting Standards Update 2014-09 [Member]
Revenue
Prepaid expenses and other current assets			(117)
Accrued income taxes			112
Deferred revenue and customer deposits			(549)
Retained earnings			$ 320		$ 786	$ 6,664

[1] Prior period amounts have been adjusted to reflect the two-for-one stock split effected in the form of a stock dividend