Shareholders' Equity Rollforward (Details) - USD ($) $ in Thousands	3 Months Ended												9 Months Ended
	Sep. 30, 2018		Jun. 30, 2018		Mar. 31, 2018		Sep. 30, 2017		Jun. 30, 2017		Mar. 31, 2017		Sep. 30, 2018	Sep. 30, 2017
Increase (Decrease) in Stockholders' Equity [Roll Forward]
As of beginning balance	$ 228,612		$ 230,264		$ 221,639		$ 236,696		$ 214,464		$ 185,548		$ 221,639	$ 185,548
Net income (loss)	16,276		(2,521)		10,295		(5,947)		21,329		28,081		24,050	43,463
All Other Changes	1,020		869		(1,670)		1,013		903		835
As of ending balance	245,908		228,612		230,264		231,762		236,696		214,464		245,908	231,762
Share-based compensation	1,000						1,000						3,472	2,707
New Accounting Pronouncement or Change in Accounting Principle, Effect of Adoption, Quantification													2,700
Common Stock
Increase (Decrease) in Stockholders' Equity [Roll Forward]
As of beginning balance	151		151		150		150		150		150		150	150
All Other Changes	0		0		1		0		0		0
As of ending balance	151		151		151		150		150		150		151	150
Paid-in Capital
Increase (Decrease) in Stockholders' Equity [Roll Forward]
As of beginning balance	195,980		195,111		194,123		192,359		191,456		190,621		194,123	190,621
All Other Changes	1,020	[1]	869	[1]	988	[1]	1,013	[2]	903	[2]	835	[2]
As of ending balance	197,000		195,980		195,111		193,372		192,359		191,456		197,000	193,372
Retained Earnings
Increase (Decrease) in Stockholders' Equity [Roll Forward]
As of beginning balance	32,481		35,002		27,366		44,114		22,785		(5,296)		27,366	(5,296)
Net income (loss)	16,276		(2,521)		10,295		(5,947)		21,329		28,081
All Other Changes	0		0		(2,659)	[1]	0		0		0
As of ending balance	48,757		32,481		35,002		38,167		44,114		22,785		48,757	38,167
Accumulated Other Comprehensive Income
Increase (Decrease) in Stockholders' Equity [Roll Forward]
As of beginning balance	0		0		0		73		73		73		0	73
As of ending balance	$ 0		$ 0		$ 0		$ 73		$ 73		$ 73		$ 0	$ 73
Time Vested Restricted Stock Units [Member]
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Shares, Issued	53,411						12,252						53,411	12,252
Performance Restricted Stock Units [Member]
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Shares, Issued	1,495												1,495

[1] Includes equity-classified share-based compensation of $3.5 million during the nine months ended September 30, 2018. During the nine months ended September 30, 2018, 53,411 and 1,495 shares of common stock were issued in connection with the vesting of certain time-vested restricted stock units (“RSUs”) and performance restricted stock units (“PRSUs”), net of shares withheld for income taxes, respectively. This also includes a write-off of $2.7 million for certain accounts receivable attributable to natural gas imbalances accounted for under the entitlements method prior to January 1, 2018, in connection with the adoption of ASC Topic 606 (see Note 5).

[2] 1 Includes equity-classified share-based compensation of $2.7 million during the nine months ended September 30, 2017 and $0.1 million from the receipt in May 2017 of proceeds attributable to the rights offering in 2016 that had been held in escrow, net of costs to register our common stock. During the nine months ended September 30, 2017, 12,252 shares of common stock were issued in connection with vesting of certain RSUs, net of shares withheld for income taxes.