Condensed Parent Company Financial Statements (Condensed Statements of Cash Flow) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Net cash flows from operating activities	$ 983.4	$ 859.6	$ 871.2
Cash flows from (used for) investing activities:
Other	(29.4)	10.3	(24.8)
Net cash flows from (used for) investing activities	(1,496.3)	(1,186.5)	(919.2)
Cash flows used for financing activities:
Common stock dividends	(288.3)	(266.5)	(247.3)
Proceeds from issuance of common stock, net	149.6	26.6	151.2
Payments to retire long-term debt	(4.6)	(313.4)	(183.0)
Net change in commercial paper	171.1	84.3	18.5
Other	(45.2)	(1.7)	6.8
Net cash flows from (used for) financing activities	532.6	329.3	(3.1)
Net increase (decrease) in cash and cash equivalents	19.7	2.4	(51.1)
Cash and cash equivalents at beginning of period	8.2	5.8	56.9
Cash and cash equivalents at end of period	27.9	8.2	5.8
Supplemental cash flows information:
Interest, net of capitalized interest	(212.6)	(192.4)	(184.8)
Income taxes, net	(11.3)	(9.8)	0.0
Parent Company [Member]
Net cash flows from operating activities	273.0	254.0	262.0
Cash flows from (used for) investing activities:
Capital contributions to consolidated subsidiaries	(290.0)	(250.0)	(165.0)
Captial repayments from consolidated subsidiaries	0.0	130.0	0.0
Net change in notes receivable from and payable to affiliates	54.0	294.0	2.0
Other	0.0	10.0	0.0
Net cash flows from (used for) investing activities	(236.0)	184.0	(163.0)
Cash flows used for financing activities:
Common stock dividends	(288.0)	(267.0)	(247.0)
Proceeds from issuance of common stock, net	150.0	27.0	151.0
Payments to retire long-term debt	0.0	(250.0)	0.0
Net change in commercial paper	103.0	52.0	(1.0)
Other	(2.0)	0.0	(2.0)
Net cash flows from (used for) financing activities	(37.0)	(438.0)	(99.0)
Net increase (decrease) in cash and cash equivalents	0.0	0.0	0.0
Cash and cash equivalents at beginning of period	0.0	0.0	0.0
Cash and cash equivalents at end of period	0.0	0.0	0.0
Supplemental cash flows information:
Interest, net of capitalized interest	(3.0)	(3.0)	(3.0)
Income taxes, net	$ 0.0	$ (37.0)	$ (9.0)