Condensed Parent Company Financial Statements (Condensed Statements of Cash Flow) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Net cash flows from operating activities	$ 527.7	$ 521.6	$ 392.8
Cash flows from (used for) investing activities:
Other	(38.2)	(28.3)	9.8
Net cash flows from (used for) investing activities	(1,066.8)	(1,033.4)	(720.2)
Cash flows used for financing activities:
Common stock dividends	(312.2)	(288.3)	(266.5)
Proceeds from issuance of common stock, net	196.6	149.6	26.6
Payments to retire long-term debt	(855.7)	(4.6)	(313.4)
Net change in commercial paper	26.0	171.1	84.3
Other	(24.0)	(45.2)	(1.7)
Net cash flows from (used for) financing activities	530.7	532.6	329.3
Net increase (decrease) in cash, cash equivalents and restricted cash	(8.4)	20.8	1.9
Cash, cash equivalents and restricted cash at beginning of period	33.9	13.1	11.2
Cash, cash equivalents and restricted cash at end of period	25.5	33.9	13.1
Supplemental cash flows information:
Interest, net of capitalized interest	(247.5)	(212.6)	(192.4)
Income taxes, net	(5.0)	(11.3)	(9.8)
Parent Company [Member]
Net cash flows from operating activities	311.0	273.0	254.0
Cash flows from (used for) investing activities:
Capital contributions to consolidated subsidiaries	(625.0)	(290.0)	(250.0)
Captial repayments from consolidated subsidiaries	0.0	0.0	130.0
Net change in notes receivable from and payable to affiliates	441.0	54.0	294.0
Other	0.0	0.0	10.0
Net cash flows from (used for) investing activities	(184.0)	(236.0)	184.0
Cash flows used for financing activities:
Common stock dividends	(312.0)	(288.0)	(267.0)
Proceeds from issuance of common stock, net	197.0	150.0	27.0
Payments to retire long-term debt	0.0	0.0	(250.0)
Net change in commercial paper	(10.0)	103.0	52.0
Other	(2.0)	(2.0)	0.0
Net cash flows from (used for) financing activities	(127.0)	(37.0)	(438.0)
Net increase (decrease) in cash, cash equivalents and restricted cash	0.0	0.0	0.0
Cash, cash equivalents and restricted cash at beginning of period	0.0	0.0	0.0
Cash, cash equivalents and restricted cash at end of period	0.0	0.0	0.0
Supplemental cash flows information:
Interest, net of capitalized interest	(4.0)	(3.0)	(3.0)
Income taxes, net	$ 5.0	$ 0.0	$ (37.0)