Income Taxes (Details) $ in Millions, ₩ in Billions	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2018 USD ($)	Jun. 30, 2017 USD ($)	Jun. 30, 2018 USD ($)	Jun. 30, 2017 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2017 KRW (₩)
Income tax provision	$ 12.4	$ 4.6	$ 19.7	$ 9.6
Effective income tax rate	29.70%	13.20%	24.60%	13.50%
Income tax expense, adjustments for ASU 2016-09, adjustment pursuant to SAB-118, and changes in valuation allowance	$ 3.9
Income tax expense, uncertain tax positions		$ 2.7
Income tax benefit, adjustment pursuant to SAB-118, uncertain tax positions, adjustments for ASU 2016-09, and changes in valuation allowance			$ 8.8
Income tax expense due to change in estimate related to uncertain tax position and adjustment for ASU 2016-09				$ 3.9
Tax Authority, South Korea (SRTO)
Income tax assessment notice, amount					$ 10.6	₩ 11.3
Without income tax benefit adjustment and valuation allowance
Effective income tax rate	39.20%	5.40%	35.60%	8.10%