Common Equity - Stock Options (Details) - Stock options - USD ($) $ / shares in Units, $ in Millions	1 Months Ended	12 Months Ended
	Jan. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Options Activity [Roll Forward]
Outstanding, shares, beginning balance	5,122,775	5,984,664
Granted, shares		794,764	516,475	899,500
Exercised, shares		(1,644,353)
Forfeited, shares		(12,300)
Outstanding, shares, ending balance		5,122,775	5,984,664
Options - Weighted Average Exercise Price
Outstanding, Weighted-Average Exercise Price, Beginning	$ 38.95	$ 33.47
Granted, Weighted-Average Exercise Price		52.15
Exercised, Weighted-Average Exercise Price		25.30
Forfeited, Weighted-Average Exercise Price		52.98
Outstanding, Weighted-Average Exercise Price, Ending		$ 38.95	$ 33.47
Options - Additional Disclosures
Outstanding, Weighted-Average Remaining Contractual Life (Years)		6 years
Outstanding, Aggregate Intrinsic Value		$ 100.9
Exercisable, shares		3,710,836
Exercisable, Weighted-Average Exercise Price		$ 35.38
Exercisable, Weighted-Average Remaining Contractual Life (Years)		5 years 2 months
Exercisable, Aggregate Intrinsic Value		$ 86.4
Intrinsic value of options exercised		55.4	$ 36.1	$ 50.5
Actual tax benefit realized for the tax deductions		$ 22.2	$ 14.5	$ 19.9
Estimated fair value per non-qualified stock option (in dollars per share)		$ 5.14	$ 5.29	$ 4.18
Subsequent event
Options Activity [Roll Forward]
Granted, shares	552,215
Options - Weighted Average Exercise Price
Granted, Weighted-Average Exercise Price	$ 58.31
Options - Additional Disclosures
Estimated fair value per non-qualified stock option (in dollars per share)	$ 7.45