Note 6 - Debt and Credit Facilities (Detail) - Debt to Capitalization Ratios	Mar. 31, 2013	Dec. 31, 2012	Mar. 31, 2012
AGL Capital [Member]
Debt-to-capitalization ratio	0.54	0.58	0.54
Nicor Gas [Member]
Debt-to-capitalization ratio	0.43	0.55	0.47