Income Taxes - Valuation Allowance (Details 3) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
SEC Schedule, 12-09, Movement in Valuation Allowances and Reserves [Roll Forward]
Balance at beginning of year	$ 693.5	$ 735.7	$ 1,189.7
Change in valuation allowance		(5.3)
Balance at end of year	659.4	693.5	735.7
Included in income tax expense (benefit) [Member]
SEC Schedule, 12-09, Movement in Valuation Allowances and Reserves [Roll Forward]
Change in valuation allowance	(26.2)	(52.8)	(62.3)
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	(7.9)	10.6	8.5
Effect of tax rate changes [Member]
SEC Schedule, 12-09, Movement in Valuation Allowances and Reserves [Roll Forward]
Change in valuation allowance	$ 0.0	$ 0.0	$ (400.2)