Fair Value Measurements - Recurring Basis (Details) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2014	Dec. 31, 2013
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Noncurrent derivative instruments	$ 5,758	$ 5,439
Total assets	35,953	22,902
Total liabilities	(1,448)	(30,302)
Recurring
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative instruments	30,195	17,463
Noncurrent derivative instruments	5,758	5,439
Total assets	35,953	22,902
Derivative instruments	(808)	(30,302)
Noncurrent derivative instruments	(640)
Total liabilities	(1,448)	(30,302)
Net derivative asset	34,505	(7,400)
Recurring | Level 2
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative instruments	16,355	(1,658)
Noncurrent derivative instruments	4,763	4,383
Total assets	21,118	2,725
Derivative instruments	(1,499)	(28,414)
Noncurrent derivative instruments	0
Total liabilities	(1,499)	(28,414)
Net derivative asset	19,619	(25,689)
Recurring | Level 3
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative instruments	13,840	19,121
Noncurrent derivative instruments	995	1,056
Total assets	14,835	20,177
Derivative instruments	691	(1,888)
Noncurrent derivative instruments	(640)
Total liabilities	51	(1,888)
Net derivative asset	$ 14,886	$ 18,289