Segment Information (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2014 segment	Mar. 31, 2013	Dec. 31, 2013
Segment Reporting Information
Number of business segments	2
Segment Reporting Information, Profit (Loss)
Operating revenues from continuing operations	$ 561,178	$ 474,016
Operating income (loss) from continuing operations	104,599	102,190
Other income (expense) from continuing operations	(16,310)	(15,087)
Income From Continuing Operations Before Income Taxes	88,289	87,103
Identifiable assets	6,780,390		6,622,212
Oil and gas operations
Segment Reporting Information, Profit (Loss)
Operating revenues from continuing operations	297,278	236,331
Operating income (loss) from continuing operations	33,548	23,181
Other income (expense) from continuing operations	(13,819)	(12,031)
Identifiable assets	5,521,883		5,379,135
Natural gas distribution
Segment Reporting Information, Profit (Loss)
Operating revenues from continuing operations	263,900	237,685
Operating income (loss) from continuing operations	72,351	79,293
Other income (expense) from continuing operations	(2,839)	(3,130)
Identifiable assets	1,209,375		1,193,413
Eliminations and corporate expenses
Segment Reporting Information, Profit (Loss)
Operating income (loss) from continuing operations	(1,300)	(284)
Other income (expense) from continuing operations	348	74
Identifiable assets	$ 49,132		$ 49,664