Financial Instruments Commodity Contract Volume (Details)	Dec. 31, 2016 MMcf
Commodity Contract Outstanding Volumes [Line Items]
Investment Contract Volume	86,609
Nondesignated [Member]
Commodity Contract Outstanding Volumes [Line Items]
Investment Contract Volume	109,012
Fair Value Hedging [Member]
Commodity Contract Outstanding Volumes [Line Items]
Investment Contract Volume	(22,403)