Income Taxes - Valuation Allowance (Details 3) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2018	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
SEC Schedule, 12-09, Movement in Valuation Allowances and Reserves [Roll Forward]
Balance at beginning of year	$ 735.7	$ 735.7	$ 1,189.7	$ 1,232.3
Change in valuation allowance	$ (5.3)
Balance at end of year		693.5	735.7	1,189.7
Included in income tax expense (benefit) [Member]
SEC Schedule, 12-09, Movement in Valuation Allowances and Reserves [Roll Forward]
Change in valuation allowance		(52.8)	(62.3)	11.0
Change in deferred assets in other comprehensive income [Member]
SEC Schedule, 12-09, Movement in Valuation Allowances and Reserves [Roll Forward]
Change in valuation allowance		10.6	8.5	(53.6)
Effect of tax rate changes [Member]
SEC Schedule, 12-09, Movement in Valuation Allowances and Reserves [Roll Forward]
Change in valuation allowance		$ 0.0	$ (400.2)	$ 0.0