Equity Method Investments (Details) (USD $)	12 Months Ended			0 Months Ended	1 Months Ended
	Oct. 31, 2014	Oct. 31, 2013	Oct. 31, 2012	Jul. 01, 2013	Sep. 30, 2013	Jun. 30, 2013
Schedule of Equity Method Investments [Line Items]
Retained Earnings, Undistributed Earnings from Equity Method Investees	$ 0
Capital contributions to or payments to acquire equity method investments	37,642,000	41,348,000	3,566,000
Cardinal Pipeline Company [Member]
Schedule of Equity Method Investments [Line Items]
Equity Method Investment, Ownership Percentage	21.49%
Equity Method Investment, Description of Principal Activities	Cardinal owns and operates an intrastate natural gas pipeline in North Carolina and is regulated by the NCUC. Cardinal has firm, long-term service agreements with local distribution companies for 100% of the firm transportation capacity on the pipeline, of which Piedmont subscribes to approximately 53%. Cardinal is dependent on the Williams – Transco pipeline system to deliver gas into its system for service to its customers.
Pipeline Subscription Capacity Percentage	100.00%
Pipeline Transportation Capacity Subscribed	53.00%
Summarized Financial Information Percentage	100.00%	100.00%	100.00%
Transportation or gas storage costs	8,825,000	8,775,000	6,613,000
Trade accounts payable	747,000	755,000
Equity Method Investment, Summarized Financial Information [Abstract]
Current assets	8,856,000	15,179,000
Noncurrent assets	111,881,000	116,414,000
Current liabilities	1,468,000	2,637,000
Noncurrent liabilities	45,402,000	45,273,000
Revenues	16,705,000	17,649,000	16,165,000
Gross profit	16,705,000	17,649,000	16,165,000
Income before income taxes	8,042,000	9,361,000	10,433,000
Pine Needle Company [Member]
Schedule of Equity Method Investments [Line Items]
Equity Method Investment, Ownership Percentage	45.00%					40.00%
Equity Method Investment, Description of Principal Activities	Pine Needle LNG Company, L.L.C. (Pine Needle), a North Carolina limited liability company, owns an interstate LNG storage facility in North Carolina and is regulated by the FERC. Pine Needle has firm, long-term service agreements for 100% of the storage capacity of the facility, of which Piedmont subscribes to approximately 64%.
Pipeline Subscription Capacity Percentage	100.00%
Pipeline Transportation Capacity Subscribed	64.00%
Capital contributions to or payments to acquire equity method investments				2,900,000
Summarized Financial Information Percentage	100.00%	100.00%	100.00%
Transportation or gas storage costs	11,364,000	11,098,000	10,410,000
Trade accounts payable	989,000	940,000
Equity Method Investment, Additional Information	Effective July 1, 2013, we acquired Hess Corporation’s 5% membership interest in Pine Needle for $2.9 million,which increased our membership interest from 40% to 45%.
Equity Method Investment, Summarized Financial Information [Abstract]
Current assets	8,812,000	9,225,000
Noncurrent assets	70,837,000	74,710,000
Current liabilities	38,029,000	3,531,000
Noncurrent liabilities	0	35,391,000
Revenues	18,025,000	16,810,000	16,390,000
Gross profit	18,025,000	16,810,000	16,390,000
Income before income taxes	6,011,000	5,804,000	5,832,000
Additional Equity Method Ownership Percentage Acquired				5.00%
South Star Energy Services [Member]
Schedule of Equity Method Investments [Line Items]
Equity Method Investment, Ownership Percentage	15.00%
Equity Method Investment, Description of Principal Activities	SouthStar primarily sells natural gas in the unregulated retail gas market to residential, commercial and industrial customers in the eastern United States, primarily in Georgia and Illinois. We account for our investment in SouthStar using the equity method, as we have board representation with equal voting rights on significant governance matters and policy decisions, and thus, exercise significant influence over the operations of SouthStar.
Capital contributions to or payments to acquire equity method investments					22,500,000
Summarized Financial Information Percentage	100.00%	100.00%	100.00%
Operating revenues	3,541,000	3,291,000	2,442,000
Trade accounts receivable	460,000	441,000
Equity Method Investment, Additional Information	In September 2013, GNGC contributed its retail natural gas marketing assets and customer accounts located in Illinois. AGL acquired these retail assets and customers from Nicor Inc. in December 2011 and additional retail natural gas assets and customer accounts in a separate transaction in June 2013. We made an additional $22.5 million capital contribution to SouthStar, maintaining our 15% equity ownership, related to this transaction.
Equity Method Investment, Summarized Financial Information [Abstract]
Current assets	196,286,000	199,425,000
Noncurrent assets	143,420,000	147,571,000
Current liabilities	51,435,000	76,346,000
Noncurrent liabilities	83,000	31,000
Revenues	845,695,000	639,426,000	585,291,000
Gross profit	234,581,000	174,993,000	161,122,000
Income before income taxes	136,569,000	102,805,000	94,631,000
Hardy Storage [Member]
Schedule of Equity Method Investments [Line Items]
Equity Method Investment, Ownership Percentage	50.00%
Equity Method Investment, Description of Principal Activities	Hardy Storage owns and operates an underground interstate natural gas storage facility located in Hardy and Hampshire Counties, West Virginia, that is regulated by the FERC. Hardy Storage has firm, long-term service agreements for 100% of the storage capacity of the facility, of which Piedmont subscribes to approximately 40%.
Summarized Financial Information Percentage	100.00%	100.00%	100.00%
Transportation or gas storage costs	9,461,000	9,702,000	9,702,000
Trade accounts payable	774,000	808,000
Storage Capacity Subscription Percentage	100.00%
Storage Capacity Subscribed	40.00%
Equity Method Investment, Summarized Financial Information [Abstract]
Current assets	12,644,000	7,641,000
Noncurrent assets	157,861,000	161,282,000
Current liabilities	17,316,000	12,378,000
Noncurrent liabilities	78,830,000	87,184,000
Revenues	23,804,000	24,375,000	24,359,000
Gross profit	23,804,000	24,375,000	24,359,000
Income before income taxes	10,497,000	10,582,000	9,939,000
Constitution Pipeline Company [Member]
Schedule of Equity Method Investments [Line Items]
Equity Method Investment, Ownership Percentage	24.00%
Equity Method Investment, Description of Principal Activities	The purpose of the joint venture is to develop, construct, own and operate approximately 120 miles of interstate natural gas pipeline and related facilities connecting shale natural gas supplies and gathering systems in Susquehanna County, Pennsylvania, to the Iroquois Gas Transmission and Tennessee Gas Pipeline systems in New York. We have committed to fund an amount in proportion to our ownership interest for the development and construction of the new pipeline, which is expected to cost approximately $680 million at the project level. As of October 31, 2014, our fiscal year contributions were $37.6 million, with our total equity contributions for the project totaling $53.5 million to date. On December 2, 2014, FERC issued a certificate of public convenience and necessity approving construction of the Constitution pipeline. The target in-service date of the project is late 2015 or 2016. The capacity of the pipeline is 100% subscribed under fifteen year service agreements with two Marcellus producer-shippers with a negotiated rate structure.
Pipeline Subscription Capacity Percentage	100.00%
Capital contributions to or payments to acquire equity method investments	37,600,000
Total Contributions To Equity Method Investments For Project	53,500,000
Summarized Financial Information Percentage	100.00%	100.00%
Estimated Pipeline Development And Construction Costs	730,000,000
Pipeline Target In Service Date	late 2015 or 2016
Equity Method Investment, Summarized Financial Information [Abstract]
Current assets	11,273,000	10,944,000
Noncurrent assets	219,208,000	62,438,000
Current liabilities	7,667,000	7,960,000
Noncurrent liabilities	0	0
Revenues	0	0
Gross profit	0	0
Income before income taxes	10,091,000	3,459,000
Atlantic Coast Pipeline [Member]
Schedule of Equity Method Investments [Line Items]
Equity Method Investment, Ownership Percentage	10.00%
Equity Method Investment, Description of Principal Activities	ACP intends to construct, operate and maintain a 550 mile natural gas pipeline, with associated compression, from West Virginia through Virginia into eastern North Carolina. The pipeline is proposed to provide interstate natural gas transportation services for Marcellus and Utica gas supplies into southeastern markets. ACP, which is regulated by the FERC, will be designed with an initial capacity of 1.5 billion cubic feet per day with a target in-service date of late 2018. The capacity of ACP is substantially subscribed by the members of ACP, other utilities and related companies under twenty-year contracts.
Capital contributions to or payments to acquire equity method investments	0
Pipeline Target In Service Date	late 2018
Long-term Purchase Commitment, Period	20 years
Equity Method Investment, Summarized Financial Information [Abstract]
Estimated Percentage Project Financing	70.00%
Atlantic Coast Pipeline [Member] | Minimum [Member]
Schedule of Equity Method Investments [Line Items]
Estimated Pipeline Development And Construction Costs	4,500,000,000
Atlantic Coast Pipeline [Member] | Maximum [Member]
Schedule of Equity Method Investments [Line Items]
Estimated Pipeline Development And Construction Costs	$ 5,000,000,000