Commitments, Guarantees and Contingencies - Narrative (Details) $ in Millions				1 Months Ended				3 Months Ended		6 Months Ended	9 Months Ended	12 Months Ended
	Oct. 01, 2015 $ / item	Aug. 07, 2014 USD ($)	Nov. 05, 2012 USD ($)	Oct. 31, 2015 USD ($)	Sep. 30, 2015 USD ($) Bcf	Sep. 30, 2014 Bcf	Jun. 30, 2013 USD ($)	Dec. 31, 2015 USD ($) $ / item	Dec. 31, 2012 USD ($)	Jun. 30, 2015 Bcf	Sep. 30, 2015 Bcf	Dec. 31, 2015 USD ($) $ / item Bcf	Dec. 31, 2014 USD ($)	Dec. 31, 2013 USD ($)	Dec. 31, 2009 USD ($)	Feb. 28, 2015 USD ($)	Feb. 28, 2012 USD ($)
Loss Contingencies [Line Items]
Commodity purchase commitment, minimum volume required (in Bcf) | Bcf												37
Long-term purchase commitment												$ 76.0
Accrued interest								$ 49.0				49.0	$ 53.0
Interest expense												173.0	179.0	$ 170.0
Operation and maintenance												914.0	$ 939.0	887.0
Gas balancing volume amount (in Bcf) | Bcf						4.6
Natural gas imbalance resolution period						5 years
Atlanta Gas Light [Member]
Loss Contingencies [Line Items]
Cumulative PRP surcharge increase (in dollars per share) | $ / item								2.44
Gas balancing volume amount (in Bcf) | Bcf					1.15						1.15
Percent of natural gas imbalance obligated (as a percent)											25.00%
Illinois Commission [Member]
Loss Contingencies [Line Items]
Loss contingency, damages sought		$ 18.0													$ 85.0
Loss contingency, estimate of possible loss																	$ 64.0
Loss contingency, damages awarded			$ 72.0				$ 72.0
Increase (decrease) in loss contingency accrual									$ 8.0
Loss contingency accrual														$ 72.0
CUB [Member]
Loss Contingencies [Line Items]
Loss contingency, damages sought		$ 22.0													305.0
Georgia Commission [Member]
Loss Contingencies [Line Items]
Gas balancing volume amount (in Bcf) | Bcf										1.15
Discretionary funds used for natural gas imbalance (as a percent)										25.00%
IAGO [Member]
Loss Contingencies [Line Items]
Loss contingency, damages sought															$ 255.0
Financial Guarantee [Member] | Tropic Equipment Leasing Inc.[Member]
Loss Contingencies [Line Items]
Maximum exposure								$ 1.0				1.0
True Up Recovery [Member] | Atlanta Gas Light [Member]
Loss Contingencies [Line Items]
Unrecovered program revenue to be recovered																$ 178.0
Interest expense												$ 1.0
PRP surcharge increase beginning Oct. 1, 2015 (in dollars per share) | $ / item	0.82
PRP surcharge increase beginning Oct. 1, 2016 (in dollars per share) | $ / item												0.81
PRP surcharge increase beginning Oct. 1, 2017 (in dollars per share) | $ / item												0.81
Operation and maintenance												$ 5.0
Capitalized mitigation costs								$ 28.0				28.0
Favorable Regulatory Action [Member] | Atlanta Gas Light [Member]
Loss Contingencies [Line Items]
Final order for rate true up				$ 144.0
PRP surcharge revenue												$ 2.0
Unfavorable Regulatory Action [Member] | Nicor Gas [Member]
Loss Contingencies [Line Items]
Loss contingency, damages sought					$ 0.3