Condensed Parent Company Financial Statements (Condensed Statements of Cash Flows) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Net cash flows from operating activities	$ 582.0	$ 501.0	$ 660.0
Cash flows from (used for) investing activities:
Dividends from consolidated subsidiaries in excess of equity earnings	(61.0)	(43.0)	(60.0)
Net cash flows from (used for) investing activities	(728.0)	(951.0)	(1,287.0)
Cash flows used for financing activities:
Common stock dividends	(403.0)	(377.0)	(337.0)
Proceeds from issuance of common stock, net	28.0	247.0	390.0
Net change in commercial paper	126.0	52.0	(104.0)
Other	(13.0)	(27.0)	(24.0)
Net cash flows from (used for) financing activities	130.0	488.0	619.0
Net increase (decrease) in cash, cash equivalents and restricted cash	(16.0)	38.0	(8.0)
Cash, cash equivalents and restricted cash at beginning of period	56.0	18.0	26.0
Cash, cash equivalents and restricted cash at end of period	40.0	56.0	18.0
Supplemental cash flows information:
Interest	(272.0)	(274.0)	(268.0)
Income taxes, net	(3.0)	5.0	21.0
Parent Company [Member]
Net cash flows from operating activities	494.0	396.0	305.0
Cash flows from (used for) investing activities:
Capital contributions to consolidated subsidiaries	(295.0)	(429.0)	(250.0)
Net change in notes receivable from and payable to affiliates	(21.0)	201.0	8.0
Dividends from consolidated subsidiaries in excess of equity earnings	50.0	0.0	0.0
Net cash flows from (used for) investing activities	(266.0)	(228.0)	(242.0)
Cash flows used for financing activities:
Common stock dividends	(403.0)	(377.0)	(338.0)
Proceeds from issuance of common stock, net	28.0	247.0	390.0
Net change in commercial paper	147.0	(37.0)	(116.0)
Other	0.0	(1.0)	1.0
Net cash flows from (used for) financing activities	(228.0)	(168.0)	(63.0)
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	(1.0)	(2.0)	(9.0)
Income taxes, net	$ 4.0	$ 10.0	$ 14.0