Common Equity - Share Repurchase Program (Details) - USD ($) shares in Millions, $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Stockholders' Equity Note [Abstract]
New shares of common stock issued	0.0	0.0	0.0
Shares purchased	1.1	1.1	1.8
Cost of shares purchased	$ 72.4	$ 71.3	$ 108.0