Income Taxes - Change in unrecognized tax benefits - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Beginning balance	$ 28.2	$ 65.2	$ 62.3
Additions for tax positions related to the current year	0.0	0.0	5.5
Additions for tax positions of prior years	4.7	1.0	0.0
Reductions for tax positions of prior years	0.0	(26.6)	(0.7)
Settlements	(23.3)	(7.1)	(1.9)
Expiration of statute of limitations	(1.3)	(4.3)	0.0
Ending balance	$ 8.3	$ 28.2	$ 65.2