Asset Retirement Obligations (Details) (USD $)	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Beginning balance	$ 296,870,000	$ 302,768,000
Accretion	31,461,000	23,052,000
New obligations	2,225,000	2,556,000
Changes in estimates of existing obligations (1)	(27,628,000)	10,895,000
Property dispositions/obligations settled	(28,941,000)	(42,401,000)
Ending balance	273,987,000	296,870,000
Annual funding obligation	36,400,000
Asset retirement obligation [Member]
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Changes in estimates of existing obligations (1)	(36,000,000)
Eminence natural gas storage caverns [Member]
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Changes in estimates of existing obligations (1)	9,000,000	13,000,000
External ARO trust [Member]
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Annual funding obligation	$ 36,400,000