New Accounting Standards - Revenue Recognition (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Jan. 02, 2018	Jan. 01, 2018	Dec. 31, 2017
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Cumulative effect of adoption, before tax	$ 7,200		$ 7,500
Prepaid expenses, deferred costs, and other current assets	106,497	$ 99,025		$ 96,106
Prepaid expenses, deferred costs, and other noncurrent assets	72,386	62,349		57,756
Deferred tax liability, net	48,812	38,709		37,130
Cumulative effect of adoption/Retained earnings	$ 122,383	112,603		$ 106,670
Accounting Standards Update 2014-09 [Member] | Difference between Revenue Guidance in Effect before and after Topic 606 [Member]
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Cumulative effect of adoption, before tax		$ 7,500
Prepaid expenses, deferred costs, and other current assets			2,919
Prepaid expenses, deferred costs, and other noncurrent assets			4,593
Deferred tax liability, net			1,579
Cumulative effect of adoption/Retained earnings			$ 5,933