Condensed Parent Company Financial Statements (Condensed Statements of Cash Flow) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Net cash flows from operating activities	$ 891.6	$ 731.0	$ 841.1
Cash flows used for investing activities:
Other	(14.9)	(18.8)	2.6
Net cash flows used for investing activities	(917.7)	(754.7)	(1,155.5)
Cash flows used for financing activities:
Common stock dividends	(225.8)us-gaap_PaymentsOfDividendsCommonStock	(208.3)us-gaap_PaymentsOfDividendsCommonStock	(199.3)us-gaap_PaymentsOfDividendsCommonStock
Proceeds from issuance of long-term debt	812.9	250.0	385.0
Payments to retire long-term debt	(358.5)us-gaap_RepaymentsOfLongTermDebt	(1.5)us-gaap_RepaymentsOfLongTermDebt	(1.4)us-gaap_RepaymentsOfLongTermDebt
Net change in commercial paper	(138.1)	11.9	164.7
Other	(7.1)	(17.4)	(8.9)
Net cash flows from (used for) financing activities	73.2	12.3	324.2
Net increase (decrease) in cash and cash equivalents	47.1	(11.4)	9.8
Cash and cash equivalents at beginning of period	9.8	21.2	11.4
Cash and cash equivalents at end of period	56.9	9.8	21.2
Supplemental cash flows information:
Interest, net of capitalized interest	(180.8)us-gaap_InterestPaidNet	(171.7)us-gaap_InterestPaidNet	(155.2)us-gaap_InterestPaidNet
Income taxes, net	5.3us-gaap_IncomeTaxesPaidNet	9.6us-gaap_IncomeTaxesPaidNet	20.3us-gaap_IncomeTaxesPaidNet
Parent Company [Member]
Net cash flows from operating activities	246.0	238.0	260.0
Cash flows used for investing activities:
Capital contributions to consolidated subsidiaries	(90.0)	(120.0)	(230.0)
Captial repayments from consolidated subsidiaries	50	95	0
Net change in notes receivable from affiliates	(23)	5	134
Other	0	(2.0)	1.0
Net cash flows used for investing activities	(63.0)	(22.0)	(95.0)
Cash flows used for financing activities:
Common stock dividends	(226.0)	(208.0)	(199.0)
Proceeds from issuance of long-term debt	250.0	0	0
Payments to retire long-term debt	(250.0)	0	0
Net change in commercial paper	45.0	(9.0)	35.0
Other	(2.0)	1.0	(1.0)
Net cash flows from (used for) financing activities	(183.0)	(216.0)	(165.0)
Net increase (decrease) in cash and cash equivalents	0	0	0
Cash and cash equivalents at beginning of period	0	0	0
Cash and cash equivalents at end of period	0	0	0
Supplemental cash flows information:
Interest, net of capitalized interest	(11.0)	(13.0)	(11.0)
Income taxes, net	$ (5.0)	$ (7.0)	$ 29.0

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