Oil and Natural Gas Operations (Unaudited) (Oil and Gas Operations) (Details)	12 Months Ended
	Dec. 31, 2015 MMBoe MBbls MMcf	Dec. 31, 2014 MMBoe MBbls MMcf	Dec. 31, 2013 MMBoe MBbls MMcf
Proved Developed and Undeveloped Reserves [Roll Forward]
Proved reserves at beginning of period, total | MMBoe	372.7	347.8	346.4
Revisions of previous estimates, total | MMBoe	(58.9)	(75.7)	4.6
Purchases, total | MMBoe	0.0	0.1	0.2
Extensions and discoveries, total | MMBoe	132.6	130.0	36.8
Production, total | MMBoe	(24.0)	(25.8)	(25.4)
Sales, total | MMBoe	(67.7)	(3.7)	(14.8)
Proved reserves at end of period, total | MMBoe	354.7	372.7	347.8
Proved developed reserves at end of period (BOE) | MMBoe	184.0	264.5	259.8
Proved undeveloped reserves at end of period (BOE) | MMBoe	170.7	108.2	88.0
Oil Reserves
Proved Developed and Undeveloped Reserves [Roll Forward]
Proved reserves at beginning of period	181,227	164,870	155,348
Revisions of previous estimates	(39,537)	(48,548)	(680)
Purchases	2	88	142
Extensions and discoveries	83,319	76,722	20,517
Production	(14,023)	(11,818)	(10,378)
Sales	(297)	(87)	(79)
Proved reserves at end of period	210,691	181,227	164,870
Proved developed reserves at end of period (volume)	108,319	118,697	113,795
Proved undeveloped reserves at end of period (volume)	102,372	62,530	51,075
Natural Gas Liquids Reserves
Proved Developed and Undeveloped Reserves [Roll Forward]
Proved reserves at beginning of period	73,463	63,011	56,155
Revisions of previous estimates	(11,979)	(15,165)	2,211
Purchases	1	26	56
Extensions and discoveries	25,530	29,695	7,823
Production	(4,065)	(4,104)	(3,233)
Sales	(11,237)	0	(1)
Proved reserves at end of period	71,713	73,463	63,011
Proved developed reserves at end of period (volume)	36,374	47,621	42,087
Proved undeveloped reserves at end of period (volume)	35,339	25,842	20,924
Natural Gas Reserves
Proved Developed and Undeveloped Reserves [Roll Forward]
Proved reserves at beginning of period | MMcf	707,926	719,725	809,128
Revisions of previous estimates | MMcf	(44,176)	(71,806)	18,465
Purchases | MMcf	2	116	282
Extensions and discoveries | MMcf	143,022	141,209	50,568
Production | MMcf	(35,604)	(59,562)	(70,506)
Sales | MMcf	(337,266)	(21,756)	(88,212)
Proved reserves at end of period | MMcf	433,904	707,926	719,725
Proved developed reserves at end of period (volume) | MMcf	236,112	589,074	623,305
Proved undeveloped reserves at end of period (volume) | MMcf	197,792	118,852	96,420