Summary of Significant Accounting Policies Accumulated Other Comprehensive Income (Loss) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012	Dec. 31, 2012
Accumulated Other Comprehensive Income (Loss), Net of Tax	$ (51.9)		$ (51.9)		$ (49.1)
Amounts reclassified from accumulated other comprehensive income (loss)	(0.9)	(0.8)	(2.8)	(2.3)
Amounts reclassified from accumulated other comprehensive income (loss)			3.3
Deconsolidation of Enogex Holdings	0	0	6.1	0
Deconsolidation of Enogex Holdings			(6.1)
Net current period other comprehensive income (loss)			(2.8)
Commodity Contract [Member]
Accumulated Other Comprehensive Income (Loss), Cumulative Changes in Net Gain (Loss) from Cash Flow Hedges, Effect Net of Tax	(0.4)		(0.4)		0.1
Amounts reclassified from accumulated other comprehensive income (loss)			0.2
Deconsolidation of Enogex Holdings			(0.7)
Net current period other comprehensive income (loss)			(0.5)
Interest Rate Swap [Member]
Accumulated Other Comprehensive Income (Loss), Cumulative Changes in Net Gain (Loss) from Cash Flow Hedges, Effect Net of Tax	(0.3)		(0.3)		(0.5)
Amounts reclassified from accumulated other comprehensive income (loss)			0.2
Deconsolidation of Enogex Holdings			0
Net current period other comprehensive income (loss)			0.2
Noncontrolling Interest [Member]
Accumulated Other Comprehensive Income (Loss), Net of Tax	0		0		(9.0)
Amounts reclassified from accumulated other comprehensive income (loss)			0.1
Deconsolidation of Enogex Holdings			8.9
Net current period other comprehensive income (loss)			9.0
Pension Plans, Defined Benefit [Member] | Defined Benefit Plan Income Loss [Member]
Accumulated Other Comprehensive Income (Loss), Pension and Other Postretirement Benefit Plans, Net of Tax	(43.7)		(43.7)		(49.3)
Amounts reclassified from accumulated other comprehensive income (loss)			2.8
Deconsolidation of Enogex Holdings			2.8
Net current period other comprehensive income (loss)			5.6
Pension Plans, Defined Benefit [Member] | Defined Benefit Plan Prior Service Cost [Member]
Accumulated Other Comprehensive Income (Loss), Pension and Other Postretirement Benefit Plans, Net of Tax	0.1		0.1		0.1
Amounts reclassified from accumulated other comprehensive income (loss)			0
Deconsolidation of Enogex Holdings			0
Net current period other comprehensive income (loss)			0
Other Postretirement Benefit Plans, Defined Benefit [Member] | Defined Benefit Plan Income Loss [Member]
Accumulated Other Comprehensive Income (Loss), Pension and Other Postretirement Benefit Plans, Net of Tax	(13.1)		(13.1)		(15.7)
Amounts reclassified from accumulated other comprehensive income (loss)			1.6
Deconsolidation of Enogex Holdings			1.0
Net current period other comprehensive income (loss)			2.6
Other Postretirement Benefit Plans, Defined Benefit [Member] | Defined Benefit Plan Prior Service Cost [Member]
Accumulated Other Comprehensive Income (Loss), Pension and Other Postretirement Benefit Plans, Net of Tax	5.5		5.5		7.2
Amounts reclassified from accumulated other comprehensive income (loss)			(1.4)
Deconsolidation of Enogex Holdings			(0.3)
Net current period other comprehensive income (loss)			$ (1.7)