Summary of Significant Accounting Policies Accumulated Other Comprehensive Income (Loss) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2013		Dec. 31, 2012	Dec. 31, 2011
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Accumulated other comprehensive (income) loss	$ (28.2)		$ (49.1)
Other comprehensive income before reclassifications	19.3
Amounts reclassified from accumulated other comprehensive income (loss)	(3.7)		(3.0)	(2.5)
Amounts reclassified from accumulated other comprehensive income (loss)	7.7	[1]
Deconsolidation of Enogex Holdings	(6.1)
Deconsolidation of Enogex Holdings	6.1		0	0
Net current period other comprehensive income (loss)	20.9
Pension settlement charges	3.0
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI on Derivatives, Tax	0.4		(1.6)	12.6
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI on Derivatives, Net of Tax	(0.6)		3.6	(27.6)
Amounts Reclassified from Accumulated OCI, Net of Tax and Noncontrolling Interest	(7.7)
Components of Net Periodic Benefit Costs to be Recognized in Next Fiscal Year	(1.0)
Pension Plan [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Amounts reclassified from accumulated other comprehensive income (loss)	(6.7)
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI, Pension and Other Postretirement Benefit Plans, for Net Gain (Loss), before Tax	(6.1)	[2]
Other Comprehensive Income Pension Settlement Adjustment	(4.9)	[2]
Other Comprehensive (Income) Loss, Reclassification Adjustment from AOCI, Pension and Other Postretirement Benefit Plans, before Tax	(11.0)
Other Comprehensive (Income) Loss, Reclassification Adjustment from AOCI, Pension and Other Postretirement Benefit Plans, Tax	(4.3)
Amortization of Defined Benefit Pension Items, Amount Reclassified from Accumulated OCI, Noncontrolling Interest	(0.1)
Amounts Reclassified from Accumulated OCI, Net of Tax and Noncontrolling Interest	(6.6)
Postretirement Benefit Plan [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Amounts reclassified from accumulated other comprehensive income (loss)	(0.2)
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI, Pension and Other Postretirement Benefit Plans, for Net Gain (Loss), before Tax	(3.3)	[2]
Other Comprehensive (Income) Loss, Reclassification Adjustment from AOCI, Pension and Other Postretirement Benefit Plans, before Tax	(0.4)
Other Comprehensive (Income) Loss, Reclassification Adjustment from AOCI, Pension and Other Postretirement Benefit Plans, Tax	(0.2)
Other Comprehensive (Income) Loss, Amortization Adjustment from AOCI, Pension and Other Postretirement Benefit Plans, for Net Prior Service Cost (Credit), before Tax	2.9	[2]
Deferred Commodity Contracts Hedging Gains Losses [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Components of Net Periodic Benefit Costs to be Recognized in Next Fiscal Year	0
Deferred Interest Rate Swap Hedging Losses [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Components of Net Periodic Benefit Costs to be Recognized in Next Fiscal Year	(0.2)
Defined Benefit Plans Income Loss [Member] | Pension Plans, Defined Benefit [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Accumulated other comprehensive income (loss)	(27.4)		(49.3)
Other comprehensive income before reclassifications	12.4
Amounts reclassified from accumulated other comprehensive income (loss)	6.7	[1]
Deconsolidation of Enogex Holdings	2.8
Net current period other comprehensive income (loss)	21.9
Components of Net Periodic Benefit Costs to be Recognized in Next Fiscal Year	(1.7)
Defined Benefit Plans Income Loss [Member] | Other Postretirement Benefit Plans, Defined Benefit [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Accumulated other comprehensive income (loss)	(5.8)		(15.7)
Other comprehensive income before reclassifications	6.9
Amounts reclassified from accumulated other comprehensive income (loss)	2.0	[1]
Deconsolidation of Enogex Holdings	1.0
Net current period other comprehensive income (loss)	9.9
Components of Net Periodic Benefit Costs to be Recognized in Next Fiscal Year	(0.8)
Defined Benefit Plan Prior Service Cost [Member] | Pension Plans, Defined Benefit [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Accumulated other comprehensive income (loss)	0.1		0.1
Other comprehensive income before reclassifications	0
Amounts reclassified from accumulated other comprehensive income (loss)	0	[1]
Deconsolidation of Enogex Holdings	0
Net current period other comprehensive income (loss)	0
Components of Net Periodic Benefit Costs to be Recognized in Next Fiscal Year	(0.1)
Defined Benefit Plan Prior Service Cost [Member] | Other Postretirement Benefit Plans, Defined Benefit [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Accumulated other comprehensive income (loss)	5.1		7.2
Other comprehensive income before reclassifications	0
Amounts reclassified from accumulated other comprehensive income (loss)	(1.8)	[1]
Deconsolidation of Enogex Holdings	(0.3)
Net current period other comprehensive income (loss)	(2.1)
Components of Net Periodic Benefit Costs to be Recognized in Next Fiscal Year	1.8
Commodity Contract [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Accumulated other comprehensive income (loss)	0		0.1
Other comprehensive income before reclassifications	0
Amounts reclassified from accumulated other comprehensive income (loss)	0.6	[1]
Deconsolidation of Enogex Holdings	(0.7)
Net current period other comprehensive income (loss)	(0.1)
Interest Rate Swap [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Accumulated other comprehensive income (loss)	(0.2)		(0.5)
Other comprehensive income before reclassifications	0
Amounts reclassified from accumulated other comprehensive income (loss)	0.3	[1]
Deconsolidation of Enogex Holdings	0
Net current period other comprehensive income (loss)	0.3
Noncontrolling Interest [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Accumulated other comprehensive (income) loss	0		(9.0)
Other comprehensive income before reclassifications	0
Amounts reclassified from accumulated other comprehensive income (loss)	0.1	[1]
Deconsolidation of Enogex Holdings	8.9
Net current period other comprehensive income (loss)	9.0
Interest Rate Swap [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI on Derivatives, before Tax	(0.4)
Commodity Contract [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI on Derivatives, before Tax	(1.0)
Derivative [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI on Derivatives, before Tax	(1.4)
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI on Derivatives, Tax	(0.5)
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI on Derivatives, Net of Tax	$ (0.9)

[1] Includes $3.0 million of pension settlement charges.

[2] These accumulated other comprehensive income (loss) components are included in the computation of net periodic benefit cost (see Note 13 for additional information).