Income Taxes (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Income Tax Expense (Benefit) [Abstract]
Current federal income tax expense (benefit)	$ 47.9	$ 33.7	$ (5.3)
Deferred federal income tax expense (benefit)	69.1	36.5	153.0
Current state income tax expense (benefit)	5.7	0.7	2.9
Deferred state income tax expense (benefit)	(2.9)	(4.4)	3.8
Total income tax expense	119.8	66.5	154.4
Deferred tax liabilities
Property, plant and equipment	1,651.1	1,606.6
Commodity price and interest rate derivatives	0	69.4
Total deferred tax liabilities	1,651.1	1,676.0
Deferred tax assets
Deferred Tax Assets, Derivatives	9.7	0
NOL and tax credit carryforwards	54.6	65.6
Employee benefits and compensation costs	36.1	47.7
Accrued litigation loss contingency	0.8	42.8
Bonus and vacation accrual	10.6	11.8
Other	9.3	9.6
Total deferred tax assets	121.1	177.5
Net deferred income tax liability	1,530.0	1,498.5
Deferred income tax liability - current	30.6	0
Deferred income tax liability -current	0	5.0
Deferred income tax liability - non-current	1,560.6	1,493.5
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	0.60%	(1.20%)	1.00%
Penalties	0.20%	(0.60%)	0.00%
Return to provision adjustment	(0.20%)	0.40%	1.30%
Noncontrolling interest	(1.40%)	(0.70%)	(0.30%)
Effective Income Tax Rate Reconciliation, Nondeductible Expense, Impairment Losses	7.20%	0.00%	0.00%
Other	(0.30%)	0.60%	(0.70%)
Effective income tax rate	41.10%	33.50%	36.30%
Domestic Tax Authority [Member]
Deferred tax assets
Deferred Tax Assets, Tax Credit Carryforwards, Alternative Minimum Tax	28.1
State and Local Jurisdiction [Member]
Effective Income Tax Rate, Continuing Operations, Tax Rate Reconciliation [Abstract]
Operating Loss Carryforwards	$ 26.5
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