Income Taxes (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Income Tax Expense (Benefit) [Abstract]
Current federal income tax expense (benefit)	$ 33.7	$ (5.3)	$ (16.6)
Deferred federal income tax expense (benefit)	36.5	153.0	172.9
Current state income tax expense (benefit)	0.7	2.9	(4.7)
Deferred state income tax expense (benefit)	(4.4)	3.8	15.4
Total income tax expense	66.5	154.4	167.0
Deferred tax liabilities
Property, plant and equipment	1,606.6	1,714.6
Commodity price and interest rate derivatives	69.4	147.2
Total deferred tax liabilities	1,676.0	1,861.8
Deferred tax assets
NOL and tax credit carryforwards	65.6	232.9
Employee benefits and compensation costs	47.7	42.9
Accrued litigation loss contingency	42.8	0
Bonus and vacation accrual	11.8	10.7
Other	9.6	5.2
Total deferred tax assets	177.5	291.7
Net deferred income tax liability	1,498.5	1,570.1
Deferred income tax liability - current	5.0	85.4
Deferred income tax liability - non-current	1,493.5	1,484.7
Deferred Tax Assets, Tax Credit Carryforwards [Abstract]
Federal income taxes statutory rate	35.00%	35.00%	35.00%
Increase (decrease) in rate as a result of:
State income taxes, net of federal income tax benefit	(1.20%)	1.00%	1.50%
Income tax rate reconciliation penalties	(0.60%)	0.00%	0.00%
Return to provision adjustment	0.40%	1.30%	0.20%
Noncontrolling interest	(0.70%)	(0.30%)	(0.20%)
Non-deductible Spin-off costs	0.00%	0.00%	0.50%
Other	0.60%	(0.70%)	(0.10%)
Effective income tax rate	33.50%	36.30%	36.90%
Domestic Tax Authority [Member]
Deferred tax assets
Deferred Tax Assets, Tax Credit Carryforwards, Alternative Minimum Tax	44.4
State and Local Jurisdiction [Member]
Effective Income Tax Rate, Continuing Operations, Tax Rate Reconciliation [Abstract]
Operating Loss Carryforwards	$ 21.2
State and Local Jurisdiction [Member] | Minimum [Member]
Effective Income Tax Rate, Continuing Operations, Tax Rate Reconciliation [Abstract]
Operating Loss Carryforwards, Expiration Dates	2014
State and Local Jurisdiction [Member] | Maximum [Member]
Effective Income Tax Rate, Continuing Operations, Tax Rate Reconciliation [Abstract]
Operating Loss Carryforwards, Expiration Dates	2032