Employee Benefits - Adoption of ASU 2017-07 (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Other operation and maintenance	$ 2,270.5	$ 2,056.1	$ 2,171.3
Other income, net	70.3	73.7	66.6
Non-service credit (cost) components of net periodic benefit costs	$ 26.0	9.1	(14.2)
2017 Form 10-K Income Statement
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Other operation and maintenance		2,047.0	2,185.5
Other income, net		64.6	80.8
Impact of ASU 2017-07
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Non-service credit (cost) components of net periodic benefit costs		$ 9.1	$ (14.2)