Condensed Parent Company Financial Statements (Condensed Statements of Cash Flow) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Net cash flows from operating activities	$ 871.2	$ 891.6	$ 731.0
Cash flows used for investing activities:
Other	(24.8)	(14.9)	(18.8)
Net cash flows used for investing activities	(919.2)	(917.7)	(754.7)
Cash flows used for financing activities:
Common stock dividends	(247.3)	(225.8)	(208.3)
Proceeds from issuance of common stock, net	151.2	0.0	0.0
Proceeds from issuance of long-term debt	250.7	812.9	250.0
Payments to retire long-term debt	(183.0)	(358.5)	(1.5)
Net change in commercial paper	18.5	(138.1)	11.9
Other	6.8	(17.3)	(28.8)
Net cash flows from (used for) financing activities	(3.1)	73.2	12.3
Net increase (decrease) in cash and cash equivalents	(51.1)	47.1	(11.4)
Cash and cash equivalents at beginning of period	56.9	9.8	21.2
Cash and cash equivalents at end of period	5.8	56.9	9.8
Supplemental cash flows information:
Interest, net of capitalized interest	(184.8)	(180.8)	(171.7)
Income taxes, net	0.0	5.3	9.6
Parent Company [Member]
Net cash flows from operating activities	262.0	246.0	238.0
Cash flows used for investing activities:
Capital contributions to consolidated subsidiaries	(165.0)	(90.0)	(120.0)
Captial repayments from consolidated subsidiaries	0.0	50.0	95.0
Net change in notes receivable from affiliates	2.0	(23.0)	5.0
Other	0.0	0.0	(2.0)
Net cash flows used for investing activities	(163.0)	(63.0)	(22.0)
Cash flows used for financing activities:
Common stock dividends	(247.0)	(226.0)	(208.0)
Proceeds from issuance of common stock, net	151.0	0.0	0.0
Proceeds from issuance of long-term debt	0.0	250.0	0.0
Payments to retire long-term debt	0.0	(250.0)	0.0
Net change in commercial paper	(1.0)	45.0	(9.0)
Other	(2.0)	(2.0)	1.0
Net cash flows from (used for) financing activities	(99.0)	(183.0)	(216.0)
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)	(11.0)	(13.0)
Income taxes, net	$ (9.0)	$ (5.0)	$ (7.0)