Fair Value Measurements (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2015		Dec. 31, 2014		Mar. 31, 2014
Assets and liabilities measured at fair value on a recurring basis
Cash equivalents	$ 1,491		$ 269		$ 25,821
Restricted cash	685		429		652
Commodity derivatives, net	29,582	[1]	25,885	[1]	87,808	[1]
Convertible preferred securities	13,300	[2]	13,300	[2]	20,530	[2]
Other assets and liabilities	8,066	[3]	8,203	[3]	10,009	[3]
Total	53,124		48,086		144,820
Level 1
Assets and liabilities measured at fair value on a recurring basis
Cash equivalents	1,491		269		25,821
Restricted cash	685		429		652
Commodity derivatives, net	60,796	[1]	72,868	[1]	82,626	[1]
Convertible preferred securities	0	[2]	0	[2]	0	[2]
Other assets and liabilities	11,583	[3]	10,869	[3]	10,960	[3]
Total	74,555		84,435		120,059
Level 2
Assets and liabilities measured at fair value on a recurring basis
Cash equivalents	0		0		0
Restricted cash	0		0		0
Commodity derivatives, net	(31,214)	[1]	(46,983)	[1]	5,182	[1]
Convertible preferred securities	0	[2]	0	[2]	0	[2]
Other assets and liabilities	(3,517)	[3]	(2,666)	[3]	(951)	[3]
Total	(34,731)		(49,649)		4,231
Level 3
Assets and liabilities measured at fair value on a recurring basis
Cash equivalents	0		0		0
Restricted cash	0		0		0
Commodity derivatives, net	0	[1]	0	[1]	0	[1]
Convertible preferred securities	13,300	[2]	13,300	[2]	20,530	[2]
Other assets and liabilities	0	[3]	0	[3]	0	[3]
Total	$ 13,300		$ 13,300		$ 20,530

[1] Includes associated cash posted/received as collateral

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

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