Income Taxes - Income Tax Expense and Income Before Income Taxes (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Taxes
Current income taxes (benefits): U.S. federal	$ (4.4)	$ 38.6	$ 225.9
Current income taxes (benefits): State	48.9	(1.9)	12.3
Current income taxes (benefits): Foreign	38.1	36.6	32.6
Current income taxes (benefits): Tax benefit of operating loss carryforwards	(21.5)	(17.5)	(52.0)
Total current income taxes (benefits)	61.1	55.8	218.8
Deferred income taxes (benefits): U.S. federal	(208.2)	171.7	79.1
Deferred income taxes (benefits): State	42.5	(20.6)	(1.0)
Deferred income taxes (benefits): Foreign	32.3	23.0	(119.3)
Deferred income taxes (benefits)	(133.4)	174.1	(41.2)
Income taxes (benefits)	(72.3)	229.9	177.6
Income (loss) before income taxes
Income (loss) before income taxes - domestic	1,848.5	1,261.4	1,195.3
Income (loss) before income taxes - foreign	403.7	330.3	235.5
Income (loss) before income taxes	$ 2,252.2	$ 1,591.7	$ 1,430.8