Supplemental Cash Flow Information (Schedule of Supplemental Cash Flow Information) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Supplemental Cash Flow Elements [Abstract]
Interest paid, net of amount capitalized	$ 40,747	$ 32,172	$ 38,255
Income taxes paid	8,114	219,505	22,781
Noncash investing activities:
Accrued development, exploration costs and other capital	79,206	207,461	93,623
Capitalized asset retirement obligations costs	981	2,958	2,772
Receivable from sale of Alabama Gas Corporation	0	8,247	0
Noncash financing activities:
Issuance of common stock for employee benefit plans	5,758	2,448	1,015
Treasury stock acquired in connection with tax withholdings	$ 4,722	$ 2,547	$ 977