Schedule I Financial Statements of Parent Company Statement of Cash Flows (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Cash Flows from Operating Activities
Net income (loss)	$ 22.9	$ 126.8	$ 44.4	$ 18.9	$ 19.5	$ 147.4	$ 52.1	$ 23.8
Adjustments to reconcile income to net cash from operating activities:
Deferred income taxes, net									$ 124.9	$ 125.8	$ 134.0
Equity in earnings from subsidiaries									(1.2)	0.0	0.2
Cash flows affected by changes in:
Accrued taxes									1.1	8.3	2.2
Accrued interest									3.4	(4.1)	3.9
Other operating activities									(2.9)	(12.3)	16.2
Net cash from operating activities									753.1	698.2	776.8
Cash Flows from Investing Activities
Other investing activities									(48.2)	(27.5)	(21.7)
Net cash from investing activities									(734.4)	(779.8)	(705.8)
Cash Flows from Financing Activities
Issuance of common stock									3.0	4.8	4.9
Issuance of long-term debt									348.8	0.0	762.5
Issuance fees									(3.0)	(0.9)	(9.0)
Repayment of long-term debt									(87.0)	(13.4)	(265.3)
Net change in short-term borrowings									(128.3)	245.1	(424.9)
Dividends paid									(155.5)	(145.6)	(137.3)
Other financing activities									(2.4)	(2.0)	(1.6)
Net cash from financing activities									(20.4)	84.0	(69.7)
Net Change in Cash and Cash Equivalents									(1.7)	2.4	1.3
Cash and Cash Equivalents at Beginning of Year				13.0				10.6	13.0	10.6	9.3
Cash and Cash Equivalents at End of Year	11.3				13.0				11.3	13.0	10.6
Parent Company [Member]
Cash Flows from Operating Activities
Net income (loss)									213.0	242.8	250.2
Adjustments to reconcile income to net cash from operating activities:
Amortization									0.8	4.8	10.6
Deferred income taxes, net									(1.7)	(1.4)	(10.5)
Equity in earnings from subsidiaries									(220.9)	(251.1)	(256.5)
Cash flows affected by changes in:
Accounts receivable from subsidiaries									(0.1)	(3.8)	(0.1)
Taxes receivable									0.0	0.2	(0.2)
Accounts payable to subsidiaries									1.3	(3.2)	(0.5)
Other accounts payable									0.0	0.0	0.1
Accrued taxes									0.3	4.3	(0.1)
Accrued interest									0.0	(0.1)	(2.6)
Cash dividends from subsidiaries									157.0	144.0	140.0
Uncertain tax positions									0.0	(2.9)	7.3
Other operating activities									8.7	11.8	6.8
Net cash from operating activities									158.4	145.4	144.5
Cash Flows from Investing Activities
Intercompany lending									(1.4)	0.0	248.7
Net money pool lending									(0.4)	6.1	(5.4)
Investment in subsidiary									(7.8)	(3.6)	(0.5)
Net cash from investing activities									(9.6)	2.5	242.8
Cash Flows from Financing Activities
Issuance of common stock									3.0	4.8	4.9
Issuance fees									0.0	(0.1)	(0.4)
Repayment of long-term debt									0.0	0.0	(250.0)
Net change in short-term borrowings									6.0	(5.0)	(3.0)
Dividends paid									(155.5)	(145.6)	(137.3)
Other financing activities									(2.3)	(2.0)	(1.5)
Net cash from financing activities									(148.8)	(147.9)	(387.3)
Net Change in Cash and Cash Equivalents									0.0	0.0	0.0
Cash and Cash Equivalents at Beginning of Year				$ 0.0				$ 0.0	0.0	0.0	0.0
Cash and Cash Equivalents at End of Year	$ 0.0				$ 0.0				$ 0.0	$ 0.0	$ 0.0