Common Equity - Restricted Shares (Details) - Restricted stock - USD ($) $ / shares in Units, $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Restricted Stock Activity
Outstanding, shares, beginning of period	134,109	234,627
Granted, shares		97,343
Released, shares		(192,291)
Forfeited, shares		(5,570)
Outstanding, shares, end of period		134,109	234,627
Restricted Stock Weighted-Average Grant Date Fair Value
Outstanding, weighted-average grant date fair value, beginning of period	$ 66.48	$ 61.01
Granted, weighted-average grant date fair value		68.18
Released, weighted-average grant date fair value		60.76
Forfeited, weighted-average grant date fair value		62.99
Outstanding, weighted-average grant date fair value, end of period		$ 66.48	$ 61.01
Restricted Stock - Additional Disclosures
Intrinsic value of released restricted shares		$ 13.4	$ 7.9	$ 5.4
Tax benefit from released restricted shares		3.7	$ 2.2	$ 2.1
Compensation cost not yet recognized		$ 2.4
Weighted-average period over which unrecognized compensation cost is expected to be recognized		1 year 7 months 6 days
Subsequent event
Restricted Stock Activity
Granted, shares	84,540
Restricted Stock Weighted-Average Grant Date Fair Value
Granted, weighted-average grant date fair value	$ 91.49