Fair Value Measurements (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2014		Dec. 31, 2013
Assets and liabilities measured at fair value on a recurring basis
Cash equivalents	$ 269		$ 97,751
Restricted cash	429		408
Commodity derivatives, net (a)	25,885	[1]	967	[1]
Convertible preferred securities	13,300	[2]	25,720	[2]
Other assets and liabilities	8,203	[3]	9,984	[3]
Total	48,086		134,830
Level 1
Assets and liabilities measured at fair value on a recurring basis
Cash equivalents	269		97,751
Restricted cash	429		408
Commodity derivatives, net (a)	72,868	[1]	50,777	[1]
Convertible preferred securities	0	[2]	0	[2]
Other assets and liabilities	10,869	[3]	10,143	[3]
Total	84,435		159,079
Level 2
Assets and liabilities measured at fair value on a recurring basis
Cash equivalents	0		0
Restricted cash	0		0
Commodity derivatives, net (a)	(46,983)	[1]	(49,810)	[1]
Convertible preferred securities	0	[2]	0	[2]
Other assets and liabilities	(2,666)	[3]	(159)	[3]
Total	(49,649)		(49,969)
Level 3
Assets and liabilities measured at fair value on a recurring basis
Cash equivalents	0		0
Restricted cash	0		0
Commodity derivatives, net (a)	0	[1]	0	[1]
Convertible preferred securities	13,300	[2]	25,720	[2]
Other assets and liabilities	0	[3]	0	[3]
Total	$ 13,300		$ 25,720

[1] Includes associated cash posted/received as collateral

[2] Recorded in “Other noncurrent assets” on the Company’s Consolidated Balance Sheets

[3] Included in other assets and liabilities are deferred compensation assets (Level 1) and interest rate derivatives (Level 2)