Income Taxes - Components of the provision for income taxes (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Current:
Federal									$ (19.1)	$ (61.1)	$ (195.9)
State									(1.5)	(1.1)	(3.2)
Foreign									5.3	4.5	4.4
Total current									(15.3)	(57.7)	(194.7)
Deferred:
Effect of tax cuts and jobs act - Deferred federal									(5.9)	(476.2)	0.0
Other - Deferred federal									49.1	121.9	294.0
Federal									43.2	(354.3)	294.0
State									14.7	(2.8)	11.0
Foreign									0.0	0.0	(3.5)
Total deferred									57.9	(357.1)	301.5
Provision	$ 17.7	$ 6.7	$ 12.5	$ 5.7	$ (457.9)	$ 23.4	$ 16.7	$ 3.0	$ 42.6	$ (414.8)	$ 106.8