Employee Benefit Plans - Narrative (Details) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Defined Benefit Plan Disclosure [Line Items]
Regulatory assets			$ 738,000,000	$ 714,000,000
Funding target (as a percent)			100.00%
Funding target, amortization period			7 years
Defined contribution plan, employer matching contribution			$ 17,000,000	17,000,000	$ 14,000,000
AGL Resources Retirement Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Required employer contribution, amount			0	0
Supplemental Employee Retirement Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Benefit obligation			$ 2,000,000	3,000,000
Welfare Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Required contributions by retiree, dental coverage, premium (as a percent)			100.00%
Benefit obligation			$ 318,000,000	334,000,000	$ 326,000,000
Scenario, Forecast [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension bands, new negotiated rates (as a percent)	2.00%	2.00%
Recoverable Pension and Other Retirement Benefit Costs [Member]
Defined Benefit Plan Disclosure [Line Items]
Regulatory assets			$ 125,000,000	$ 122,000,000