Common Equity - Stock Options (Details) - Stock options - USD ($) $ / shares in Units, $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Options Activity
Outstanding, shares, beginning balance	4,452,533	4,644,214
Granted, shares		710,710	552,215	794,764
Exercised, shares		(899,391)
Forfeited, shares		(3,000)
Outstanding, shares, ending balance		4,452,533	4,644,214
Options - Weighted Average Exercise Price
Outstanding, Weighted-Average Exercise Price, Beginning	$ 48.86	$ 43.11
Granted, Weighted-Average Exercise Price		65.59
Exercised, Weighted-Average Exercise Price		32.39
Forfeited, Weighted-Average Exercise Price		57.99
Outstanding, Weighted-Average Exercise Price, Ending		$ 48.86	$ 43.11
Options - Additional Disclosures
Outstanding, Weighted-Average Remaining Contractual Life (Years)		6 years 1 month
Outstanding, Aggregate Intrinsic Value		$ 90.8
Exercisable, shares		2,838,609
Exercisable, Weighted-Average Exercise Price (in dollars per share)		$ 42.77
Exercisable, Weighted-Average Remaining Contractual Life (Years)		4 years 11 months
Exercisable, Aggregate Intrinsic Value		$ 75.2
Intrinsic value of options exercised		32.4	$ 33.8	$ 55.4
Tax benefit from option exercises		8.9	$ 13.5	$ 22.2
Compensation cost not yet recognized		$ 3.0
Weighted-average period over which unrecognized compensation cost is expected to be recognized		1 year 8 months
Estimated weighted-average fair value per stock option (in dollars per share)		$ 7.71	$ 7.45	$ 5.14
Subsequent event
Options Activity
Granted, shares	476,418
Options - Weighted Average Exercise Price
Granted, Weighted-Average Exercise Price	$ 68.18
Options - Additional Disclosures
Estimated weighted-average fair value per stock option (in dollars per share)	$ 8.60