Segment Information (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2013 segment	Mar. 31, 2012	Dec. 31, 2012
Segment Reporting Information
Number of business segments	2
Segment Reporting Information, Profit (Loss)
Operating revenues	$ 492,679	$ 418,444
Operating income (loss)	105,336	104,170
Other income (expense)	(15,065)	(13,506)
Income Before Income Taxes	90,271	90,664
Identifiable assets	6,293,178		6,175,890
Oil and gas operations
Segment Reporting Information, Profit (Loss)
Operating revenues	254,994	223,957
Operating income (loss)	26,327	26,005
Other income (expense)	(11,993)	(10,558)
Identifiable assets	5,100,983		4,975,170
Natural gas distribution
Segment Reporting Information, Profit (Loss)
Operating revenues	237,685	194,487
Operating income (loss)	79,293	78,560
Other income (expense)	(3,130)	(3,004)
Identifiable assets	1,155,950		1,177,134
Eliminations and corporate expenses
Segment Reporting Information, Profit (Loss)
Operating income (loss)	(284)	(395)
Other income (expense)	58	56
Identifiable assets	$ 36,245		$ 23,586