Summary of Significant Accounting Policies Accumulated Other Comprehensive Income (Loss) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013	Dec. 31, 2013
Accumulated Other Comprehensive Income (Loss), Net of Tax	$ (27.6)		$ (27.6)		$ (28.2)
Amounts reclassified from accumulated other comprehensive income (loss)	(0.5)	(1.0)	(0.9)	(1.9)
Amounts reclassified from accumulated other comprehensive income (loss)	(0.3)		(0.6)
Amounts reclassified from accumulated other comprehensive income (loss)			0.6
Deconsolidation of Enogex Holdings	0	6.1	0	6.1
Net current period other comprehensive income (loss)			0.6
Interest Rate Swap [Member]
Accumulated Other Comprehensive Income (Loss), Cumulative Changes in Net Gain (Loss) from Cash Flow Hedges, Effect Net of Tax	(0.1)		(0.1)		(0.2)
Amounts reclassified from accumulated other comprehensive income (loss)			0.1
Net current period other comprehensive income (loss)			0.1
Pension Plans, Defined Benefit [Member]
Amounts reclassified from accumulated other comprehensive income (loss)	(0.5)		(0.9)
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	(26.5)		(26.5)		(27.4)
Amounts reclassified from accumulated other comprehensive income (loss)			0.9
Net current period other comprehensive income (loss)			0.9
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
Net current period other comprehensive income (loss)			0
Postretirement Benefit Plan [Member]
Amounts reclassified from accumulated other comprehensive income (loss)	0.2		0.4
Postretirement Benefit Plan [Member] | Defined Benefit Plan Income Loss [Member]
Accumulated Other Comprehensive Income (Loss), Pension and Other Postretirement Benefit Plans, Net of Tax	(5.3)		(5.3)		(5.8)
Amounts reclassified from accumulated other comprehensive income (loss)			0.5
Net current period other comprehensive income (loss)			0.5
Postretirement Benefit Plan [Member] | Defined Benefit Plan Prior Service Cost [Member]
Accumulated Other Comprehensive Income (Loss), Pension and Other Postretirement Benefit Plans, Net of Tax	4.2		4.2		5.1
Amounts reclassified from accumulated other comprehensive income (loss)			(0.9)
Net current period other comprehensive income (loss)			$ (0.9)