Condensed Parent Company Financial Statements (Condensed Statements of Cash Flows) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Net cash flows from operating activities	$ 501.0	$ 660.0	$ 528.0
Cash flows used for investing activities:
Other	(43.0)	(60.0)	(38.0)
Net cash flows used for investing activities	(951.0)	(1,287.0)	(1,067.0)
Cash flows used for financing activities:
Common stock dividends	(377.0)	(337.0)	(312.0)
Proceeds from issuance of common stock, net	247.0	390.0	197.0
Net change in commercial paper	52.0	(104.0)	26.0
Other	(27.0)	(24.0)	(24.0)
Net cash flows from (used for) financing activities	488.0	619.0	531.0
Net increase (decrease) in cash, cash equivalents and restricted cash	38.0	(8.0)	(8.0)
Cash, cash equivalents and restricted cash at beginning of period	18.0	26.0	34.0
Cash, cash equivalents and restricted cash at end of period	56.0	18.0	26.0
Supplemental cash flows information:
Interest	(274.0)	(268.0)	(247.0)
Income taxes, net	5.0	21.0	(5.0)
Parent Company [Member]
Net cash flows from operating activities	396.0	305.0	311.0
Cash flows used for investing activities:
Capital contributions to consolidated subsidiaries	(429.0)	(250.0)	(625.0)
Net change in notes receivable from and payable to affiliates	201.0	8.0	441.0
Net cash flows used for investing activities	(228.0)	(242.0)	(184.0)
Cash flows used for financing activities:
Common stock dividends	(377.0)	(338.0)	(312.0)
Proceeds from issuance of common stock, net	247.0	390.0	197.0
Net change in commercial paper	(37.0)	(116.0)	(10.0)
Other	(1.0)	1.0	(2.0)
Net cash flows from (used for) financing activities	(168.0)	(63.0)	(127.0)
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	(2.0)	(9.0)	(4.0)
Income taxes, net	$ 10.0	$ 14.0	$ 5.0