Debt and Credit Facilities - Debt-to-Capitalization Ratios (Details)	Dec. 31, 2015	Dec. 31, 2014
Schedule of Capitalization [Line Items]
Debt-to-capitalization ratio (as a percent)	0.7
AGL Resources Inc [Member]
Schedule of Capitalization [Line Items]
Debt-to-capitalization ratio (as a percent)	0.54	0.55
Nicor Gas [Member]
Schedule of Capitalization [Line Items]
Debt-to-capitalization ratio (as a percent)	0.56	0.62