Condensed Parent Company Financial Statements (Condensed Statements of Cash Flow) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015		Dec. 31, 2014
Net cash flows from operating activities	$ 859.6	$ 871.2		$ 891.6
Cash flows from (used for) investing activities:
Other	10.3	(24.8)		(14.9)
Net cash flows from (used for) investing activities	(1,186.5)	(919.2)		(917.7)
Cash flows used for financing activities:
Common stock dividends	(266.5)	(247.3)		(225.8)
Proceeds from issuance of common stock, net	26.6	151.2	[1]	0.0
Proceeds from issuance of long-term debt	800.0	250.7		812.9
Payments to retire long-term debt	(313.4)	(183.0)		(358.5)
Net change in commercial paper	84.3	18.5		(138.1)
Other	(1.7)	6.8		(17.3)
Net cash flows from (used for) financing activities	329.3	(3.1)		73.2
Net increase (decrease) in cash and cash equivalents	2.4	(51.1)		47.1
Cash and cash equivalents at beginning of period	5.8	56.9		9.8
Cash and cash equivalents at end of period	8.2	5.8		56.9
Supplemental cash flows information:
Interest, net of capitalized interest	(192.4)	(184.8)		(180.8)
Income taxes, net	(9.8)	0.0		5.3
Parent Company [Member]
Net cash flows from operating activities	254.0	262.0		246.0
Cash flows from (used for) investing activities:
Capital contributions to consolidated subsidiaries	(250.0)	(165.0)		(90.0)
Captial repayments from consolidated subsidiaries	130.0	0.0		50.0
Net change in notes receivable from and payable to affiliates	294.0	2.0		(23.0)
Other	10.0	0.0		0.0
Net cash flows from (used for) investing activities	184.0	(163.0)		(63.0)
Cash flows used for financing activities:
Common stock dividends	(267.0)	(247.0)		(226.0)
Proceeds from issuance of common stock, net	27.0	151.0		0.0
Proceeds from issuance of long-term debt	0.0	0.0		250.0
Payments to retire long-term debt	(250.0)	0.0		(250.0)
Net change in commercial paper	52.0	(1.0)		45.0
Other	0.0	(2.0)		(2.0)
Net cash flows from (used for) financing activities	(438.0)	(99.0)		(183.0)
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)		(11.0)
Income taxes, net	$ (37.0)	$ (9.0)		$ (5.0)

[1] Amounts reflect the effects of a two-for-one common stock split distributed in May 2016.