Summary of Significant Accounting Policies (Tables)

12 Months Ended

Dec. 31, 2015

Summary of Significant Accounting Policies [Abstract]

Schedule of Error Corrections and Prior Period Adjustments [Table Text Block]

	Year Ended December 31, 2014
	As reported		As revised		Change
	(in millions)
REVENUES
Purchased gas and oil sales	$	1,035.0	$	913.9	$	(121.1	)
Total Revenues	3,414.3		3,293.2		(121.1		)
OPERATING EXPENSES
Purchased gas and oil expense	$	1,031.2	$	910.1	$	(121.1	)
Total Operating Expenses	4,113.0		3,991.9		(121.1		)

	Eliminations as Reported			Eliminations as Revised			Change
	(in millions)
From unaffiliated customers	$	—		$	(121.1	)	$	(121.1	)
Purchased gas and oil expense	(1,475.4		)	(1,596.5		)	(121.1		)

Supplemental Cash Flow Information [Table Text Block]

Revision of Financial Statements

In the fourth quarter of 2015, the Company determined that certain transactions that had been reported on a gross basis and included in "Purchased gas and oil sales" and "Purchased gas and oil expense" on the Consolidated Statement of Operations for certain periods in 2014 and the first three quarters of 2015 should have been reported net, as the transactions were with the same counterparty and were entered into in contemplation of one another. The Company revised its financial statements to reflect the net accounting treatment and assessed the cumulative impact of the revisions on each period affected. The revisions had no effect on the Company’s operating income, net income, earnings per share or cash flows. The Company determined that the impact of the change from gross to net accounting was not material, either individually or in the aggregate, to previously issued financial statements. The Company has, however, recast its Consolidated Statement of Operations for the year ended December 31, 2014, to report “Purchased gas and oil sales” and “Purchased gas and oil expense” on a net basis to conform to presentation for the year ended December 31, 2015.

The following table details the impact of these revisions for the year ended December 31, 2014, on the Consolidated Statement of Operations.

	Year Ended December 31, 2014
	As reported		As revised		Change
	(in millions)
REVENUES
Purchased gas and oil sales	$	1,035.0	$	913.9	$	(121.1	)
Total Revenues	3,414.3		3,293.2		(121.1		)
OPERATING EXPENSES
Purchased gas and oil expense	$	1,031.2	$	910.1	$	(121.1	)
Total Operating Expenses	4,113.0		3,991.9		(121.1		)

Property, Plant and Equipment [Table Text Block]

Buildings	10 to 30 years
Leasehold improvements	3 to 10 years
Service, transportation and field service equipment	3 to 7 years
Furniture and office equipment	3 to 7 years

Schedule of Earnings Per Share, Basic and Diluted [Table Text Block]

	December 31,
	2015	2014	2013
	(in millions)
Weighted-average basic common shares outstanding	176.6	179.8	179.2
Potential number of shares issuable upon exercise of in-the-money stock options under the Long-Term Stock Incentive Plan	—	—	0.3
Average diluted common shares outstanding	176.6	179.8	179.5