Income Taxes - Valuation Allowance (Details 3) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Movement in Valuation Allowances and Reserves [Roll Forward]
Balance at beginning of year	$ 1,175.2	$ 1,215.5	$ 1,000.4
Balance at end of year	727.7	1,175.2	1,215.5
Included in income tax expense (benefit) [Member]
Movement in Valuation Allowances and Reserves [Roll Forward]
Change in valuation allowance	(62.8)	8.8	228.6
Change in deferred assets in other comprehensive income [Member]
Movement in Valuation Allowances and Reserves [Roll Forward]
Change in valuation allowance	11.0	(49.1)	(13.5)
Effect of tax rate changes [Member]
Movement in Valuation Allowances and Reserves [Roll Forward]
Change in valuation allowance	$ (395.7)	$ 0.0	$ 0.0