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Target allocation maximum percentage of investments in cash securities to total plan assets presented on a weighted-average basis as of the measurement date of the latest statement of financial position.

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Target allocation minimum percentage of investments in cash securities to total plan assets presented on a weighted-average basis as of the measurement date of the latest statement of financial position.

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Upper limit in the percentage range of the participant's contribution the Company matches.

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Target allocation maximum percentage of investments in debt securities to total plan assets presented on a weighted-average basis as of the measurement date of the latest statement of financial position.

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Target allocation maximum percentage of investments in equity securities to total plan assets presented on a weighted-average basis as of the measurement date of the latest statement of financial position.

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Target allocation maximum percentage in other than debt securities, equity securities, and real estate to total plan assets presented on a weighted-average basis as of the measurement date of the latest statement of financial position.

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The amount of the cost recognized during the period for defined contribution plans.

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Benefits 11 Narrative (Details) (USD $)	12 Months Ended				12 Months Ended		12 Months Ended
Benefits 11 Narrative (Details) (USD $)	Dec. 31, 2011 Missouri Gas Energy [Member]	Dec. 31, 2011 New England Gas Company [Member]	Dec. 31, 2011 Pension Plans Defined Benefit [Member]	Dec. 31, 2012 Pension Plans Defined Benefit [Member]	Dec. 31, 2011 Other Postretirement Benefit Plans Defined Benefit [Member]	Dec. 31, 2012 Other Postretirement Benefit Plans Defined Benefit [Member]	Dec. 31, 2011 Savings plan [Member]	Dec. 31, 2010 Savings plan [Member]	Dec. 31, 2009 Savings plan [Member]	Dec. 31, 2011 Retirement power accounts [Member]	Dec. 31, 2010 Retirement power accounts [Member]	Dec. 31, 2009 Retirement power accounts [Member]
Benefits [Line Items]
Net actuarial loss (gain) amortized to net periodic benefit cost			$ 10,300,000		$ 1,300,000
Net prior service cost (credit) amortized to net periodic benefit cost			580,000		(1,100,000)
Equity allocation minimum			25.00%		25.00%
Equity allocation maximum			70.00%		35.00%
Fixed income minimum			15.00%		65.00%
Fixed income maximum			35.00%		75.00%
Alternative assets minimum			10.00%
Alternative assets maximum			35.00%
Cash minimum			0.00%		0.00%
Cash maximum			10.00%		10.00%
Defined Benefit Plan, Estimated Future Employer Contributions in Next Fiscal Year				16,800,000		10,900,000
Matching contribution minimum									2.00%	3.50%
Matching contribution maximum							6.25%			12.00%
Company contributions vested	100.00%	100.00%					100.00%			100.00%
Company contributions years	six	six					five			five
Defined Contribution Plan, Cost Recognized							$ 7,600,000	$ 7,400,000	$ 7,000,000	$ 8,100,000	$ 7,900,000	$ 7,900,000