Common Equity - Stock Options (Details) - Stock options - USD ($) $ / shares in Units, $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Options Activity
Outstanding, shares, beginning balance	3,249,918	4,452,533
Granted, shares		476,418	710,710	552,215
Exercised, shares		(1,609,948)
Forfeited, shares		(69,085)
Outstanding, shares, ending balance		3,249,918	4,452,533
Options - Weighted Average Exercise Price
Outstanding, Weighted-Average Exercise Price, Beginning	$ 54.98	$ 48.86
Granted, Weighted-Average Exercise Price		68.18
Exercised, Weighted-Average Exercise Price		41.63
Forfeited, Weighted-Average Exercise Price		62.33
Outstanding, Weighted-Average Exercise Price, Ending		$ 54.98	$ 48.86
Options - Additional Disclosures
Outstanding, Weighted-Average Remaining Contractual Life (Years)		6 years 3 months 18 days
Outstanding, Aggregate Intrinsic Value		$ 121.0
Exercisable, shares		1,744,386
Exercisable, Weighted-Average Exercise Price (in dollars per share)		$ 46.92
Exercisable, Weighted-Average Remaining Contractual Life (Years)		4 years 9 months 18 days
Exercisable, Aggregate Intrinsic Value		$ 79.0
Intrinsic value of options exercised		62.4	$ 32.4	$ 33.8
Tax benefit from option exercises		17.1	$ 8.9	$ 13.5
Compensation cost not yet recognized		$ 2.1
Weighted-average period over which unrecognized compensation cost is expected to be recognized		1 year 7 months 6 days
Estimated weighted-average fair value per stock option (in dollars per share)		$ 8.60	$ 7.71	$ 7.45
Subsequent event
Options Activity
Granted, shares	512,139
Options - Weighted Average Exercise Price
Granted, Weighted-Average Exercise Price	$ 91.49
Options - Additional Disclosures
Estimated weighted-average fair value per stock option (in dollars per share)	$ 10.82