Postretirement Benefit Plans - Change in Plan Assets (Details) - Postretirement Benefit Plans [Member] - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Change in plan assets:
Fair value of plan assets at beginning of year	$ 102.4	$ 112.1
Return on plan assets	12.4	9.7
Actual return on plan assets	9.3	6.1
Defined Benefit Plan, Plan Assets, Benefits Paid	31.7	25.5
Fair value of plan assets at end of year	92.4	102.4
Funded status at end of year	$ (127.4)	$ (139.0)