Discontinued Operations and Held for Sale Properties - Balance Sheet and Income Statement (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014	Dec. 31, 2014
Assets of Disposal Group, Including Discontinued Operation [Abstract]
Total liabilities held for sale	$ 0		$ (24,230)
Disposal Group, Including Discontinued Operation, Income Statement Disclosures [Abstract]
Natural gas distribution revenues		263,900
Oil and natural gas revenues		4,812
Total revenues		268,712
Pretax income from discontinued operations		62,316
Income tax expense		23,597
Income From Discontinued Operations		38,719
Loss on disposal of discontinued operations		(1,667)
Income tax benefit		(617)
Loss on Disposal of Discontinued Operations	0	(1,050)
Income From Discontinued Operations	0	37,669
Diluted Earnings Per Average Common Share
Income from discontinued operations (in dollars per share)		$ 0.53
Loss on disposal of discontinued operations (in dollars per share)		$ (0.01)
Total Income (Loss) From Discontinued Operations (in dollars per share)	$ 0.00	$ 0.52
Basic Earnings Per Average Common Share
Income from discontinued operations (in dollars per share)		$ 0.53
Loss on disposal of discontinued operations (in dollars per share)		$ (0.01)
Total Income (Loss) From Discontinued Operations (in dollars per share)	$ 0.00	$ 0.52
San Juan Basin
Assets of Disposal Group, Including Discontinued Operation [Abstract]
Oil and natural gas properties			1,166,124
Less accumulated depreciation, depletion and amortization			(770,327)
Total assets held for sale			395,797
Other long-term liabilities			(24,230)
Total liabilities held for sale			(24,230)
Total net assets held for sale			$ 371,567