Note 7 - Debt (Detail) - Debt to Capitalization Ratios	Mar. 31, 2012	Dec. 31, 2011	Mar. 31, 2011
AGL Capital [Member]
Debt-to-capitalization ratio	54.00%	58.00%	51.00%
Nicor Gas [Member]
Debt-to-capitalization ratio	47.00%	60.00%