Summary of Significant Accounting Policies - New Accounting Pronouncements (Details) - USD ($) $ / shares in Units, $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2017	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Jan. 01, 2018	Dec. 31, 2014
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Other expense, net		$ (4,535)	$ (3,101)	$ (2,577)
Selling, general and administrative expenses		331,172	340,742	330,034
Cash provided by (used in) operating activities		29,339	39,823	70,731
Retained earnings		195,143	174,967
Accounting Standards Update 2017-07
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Other expense, net	$ 500		500	400
Selling, general and administrative expenses		(500)	(500)	(400)
Accounting Standards Update 2016-09
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Excess tax benefit, amount		$ 800
Excess tax benefit, percent		(1.20%)
Effect of change on diluted earnings per share (in dollars per share)		$ 0.03
Cash provided by (used in) operating activities		$ 800	400	600
Accounting Standards Update 2015-17
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Deferred tax asset, net – current			(4,800)
Deferred tax assets, net, noncurrent			4,800
Accounting Standards Update 2014-09 | Difference between Revenue Guidance in Effect before and after Topic 606 | Subsequent Event
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Retained earnings					$ 200
Retained Earnings
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative effect upon adoption of new accounting standard			(469)	$ 0		$ 0
Retained Earnings | Accounting Standards Update 2016-09
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative effect upon adoption of new accounting standard			500
Deferred tax assets, net			$ 300