Pension and Other Postretirement Benefit Plans - Recurring Fair Value Measures, Activity in Level 3 (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015
Pension [Member] | Private Equity Funds [Member]
Activity in Level 3 [Roll Forward]
Beginning Balance	$ 43.3	$ 43.3
Actual Return on Plan Assets	5.0	2.6
Purchases, Sales, and Settlements – Net	(7.7)	(2.6)
Ending Balance	40.6	43.3
Pension [Member] | Real Estate [Member]
Activity in Level 3 [Roll Forward]
Beginning Balance	28.9	28.9
Actual Return on Plan Assets	2.3	2.9
Purchases, Sales, and Settlements – Net	(5.6)	(2.9)
Ending Balance	25.6	28.9
Postretirement Health and Life [Member] | Private Equity Funds [Member]
Activity in Level 3 [Roll Forward]
Beginning Balance	12.0	12.9
Actual Return on Plan Assets	1.4	1.2
Purchases, Sales, and Settlements – Net	(3.9)	(2.1)
Ending Balance	$ 9.5	$ 12.0