Income Taxes (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Income Taxes [Line Items]
Effective income tax rate	18.30%	22.80%	21.80%	24.60%
Income before income taxes, domestic operations			$ 200	$ 9,200
Income before income taxes, foreign operations			64,100	66,700
Tax Cuts and Jobs Act, Incomplete Accounting, Change in Tax Rate, Deferred Tax Liability, Provisional Income Tax Benefit					$ 33,300
Tax Cuts and Jobs Act, Incomplete Accounting, Provisional Income Tax Expense (Benefit)					49,200
Liabilities for uncertain income tax positions	$ 54,500		54,500		52,200
Cash paid for income taxes			16,600	$ 26,500
Prepaid tax payments	$ 8,800		$ 8,800		$ 6,000
California Franchise Tax Board [Member]
Income Taxes [Line Items]
Income Tax Examination, Year under Examination									2013	2012
Internal Revenue Service (IRS) [Member]
Income Taxes [Line Items]
Income Tax Examination, Year under Examination						2016	2015	2014	2013	2012
Massachusetts Department of revenue [Member]
Income Taxes [Line Items]
Income Tax Examination, Year under Examination							2015	2014
New York State Division of Taxation and Finance [Member]
Income Taxes [Line Items]
Income Tax Examination, Year under Examination								2014	2013	2012	2011
Georgia Department of Revenue [Member]
Income Taxes [Line Items]
Income Tax Examination, Year under Examination						2016	2015	2014
Ministry of the Economy, Finance and Industry, France [Member]
Income Taxes [Line Items]
Income Tax Examination, Year under Examination						2016	2015	2014	2013	2012	2011