Note 5 - Derivative Instruments (Details) - Net Long Natural Gas Contracts	Mar. 31, 2015 ft3		Dec. 31, 2014 ft3		Mar. 31, 2014 ft3
Note 5 - Derivative Instruments (Details) - Net Long Natural Gas Contracts [Line Items]
Cash flow hedges	9,000,000,000	[1],[2]	9,000,000,000	[1]	6,000,000,000	[1]
Not designated as hedges	231,000,000,000	[1],[2]	75,000,000,000	[1]	277,000,000,000	[1]
Total volumes	240,000,000,000	[1],[2]	84,000,000,000	[1]	283,000,000,000	[1]
Net long position	240,000,000,000	[1],[2]	84,000,000,000	[1]	283,000,000,000	[1]
Designated as Hedging Instrument [Member]
Note 5 - Derivative Instruments (Details) - Net Long Natural Gas Contracts [Line Items]
Short position	(6,000,000,000)	[1],[2]	(7,000,000,000)	[1]	(2,000,000,000)	[1]
Long position	15,000,000,000	[1],[2]	16,000,000,000	[1]	8,000,000,000	[1]
Not Designated as Hedging Instrument [Member]
Note 5 - Derivative Instruments (Details) - Net Long Natural Gas Contracts [Line Items]
Short position	(2,735,000,000,000)	[1],[2]	(2,825,000,000,000)	[1]	(2,489,000,000,000)	[1]
Long position	2,966,000,000,000	[1],[2]	2,900,000,000,000	[1]	2,766,000,000,000	[1]

[1] Volumes related to Nicor Gas exclude variable-priced contracts, which are carried at fair value, but whose fair values are not directly impacted by changes in commodity prices.

[2] Approximately 96% of these contracts have durations of two years or less and the remaining 4% expire between two and five years.