Derivatives (Gross Current and Noncurrent Assets and Liabilities) (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017	Sep. 30, 2017
Derivatives, Fair Value [Line Items]
Commodity derivative assets	$ 36,653	$ 17,252	$ (999)
Commodity derivative liabilities	0	0	0
Commodity derivative assets - current	76,861	30,702	45,202
Commodity derivative assets - noncurrent	766	310	245
Commodity derivative liabilities - current	(91,403)	(29,651)	(38,578)
Commodity derivative liabilities - noncurrent	(2,548)	(825)	(902)
Commodity
Derivatives, Fair Value [Line Items]
Commodity derivative assets	71,493	37,730	34,842
Commodity derivative liabilities	(102,759)	(38,545)	(56,612)
Cash collateral	14,942	1,351	27,737
Total	(16,324)	536	5,967
Commodity | Commodity Derivative Assets - Current
Derivatives, Fair Value [Line Items]
Commodity derivative assets	69,957	36,929	33,804
Commodity derivative liabilities	(8,038)	(7,578)	(16,339)
Cash collateral	14,942	1,351	27,737
Commodity derivative assets - current	76,861	30,702	45,202
Commodity | Commodity Derivative Assets - Noncurrent
Derivatives, Fair Value [Line Items]
Commodity derivative assets	767	311	288
Commodity derivative liabilities	(1)	(1)	(43)
Cash collateral	0	0	0
Commodity derivative assets - noncurrent	766	310	245
Commodity | Commodity Derivative Liabilities - Current
Derivatives, Fair Value [Line Items]
Commodity derivative assets	731	489	676
Commodity derivative liabilities	(92,134)	(30,140)	(39,254)
Cash collateral	0	0	0
Commodity derivative liabilities - current	(91,403)	(29,651)	(38,578)
Commodity | Commodity Derivative Liabilities - Noncurrent
Derivatives, Fair Value [Line Items]
Commodity derivative assets	38	1	74
Commodity derivative liabilities	(2,586)	(826)	(976)
Cash collateral	0	0	0
Commodity derivative liabilities - noncurrent	$ (2,548)	$ (825)	$ (902)