Income Taxes (Details) $ in Millions, ₩ in Billions	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2018 USD ($)	Sep. 30, 2017 USD ($)	Sep. 30, 2018 USD ($)	Sep. 30, 2017 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2017 KRW (₩)
Income tax provision	$ 23.0	$ 82.6	$ 42.7	$ 92.2
Effective income tax rate	37.30%	185.00%	30.10%	79.80%
Income tax expense, adjustments for ASU 2016-09, adjustment pursuant to SAB-118, and changes in valuation allowance	$ 1.5
Effective income tax rate reconciliation, valuation allowance on U.S. deferred tax assets		$ 76.9		$ 76.9
Income tax expense, uncertain tax positions		1.7
Income tax benefit, adjustment pursuant to SAB-118, uncertain tax positions, adjustments for ASU 2016-09, and changes in valuation allowance			$ 7.4
Income tax expense due to change in estimate related to uncertain tax position and adjustment for ASU 2016-09				$ 5.6
Tax Authority, South Korea (SRTO)
Income tax assessment notice, amount					$ 10.2	₩ 11.3
2017
Effective income tax rate reconciliation, valuation allowance on U.S. deferred tax assets		24.0
2016 and previous
Effective income tax rate reconciliation, valuation allowance on U.S. deferred tax assets		$ 52.9
Without income tax benefit adjustment and valuation allowance
Effective income tax rate	34.90%	12.50%	35.30%	8.30%