Risk Management And Energy Marketing Activities (Narrative) (Details) (USD $) In Millions, unless otherwise specified	0 Months Ended		6 Months Ended	12 Months Ended			6 Months Ended	12 Months Ended	6 Months Ended	12 Months Ended			3 Months Ended
	Sep. 16, 2005	Jul. 22, 2003	Jun. 30, 2012	Dec. 31, 2011	Apr. 05, 2012	Nov. 03, 2011	Jun. 30, 2012 Physical Sales Contracts Marked-To-Market [Member]	Dec. 31, 2011 Physical Sales Contracts Marked-To-Market [Member]	Jun. 30, 2012 Financial Derivative Contracts Marked To Market [Member]	Dec. 31, 2011 Financial Derivative Contracts Marked To Market [Member]	Sep. 16, 2005 5.25% - Due September 15, 2017 [Member]	Sep. 16, 2005 5.45% - Due September 15, 2020 [Member]	Jun. 30, 2012 Millennium Pipeline Company, L.L.C [Member]	Nov. 03, 2011 MF Global [Member]	Nov. 03, 2011 Court Designated Replacement Broker [Member]	Jun. 30, 2012 Unregulated Natural Gas Business [Member]
Derivative Instruments Gain Loss [Line Items]
Reserves against physical sale contract derivatives			$ 1.6	$ 25.6
Fair value of physical sales contracts and derivative contracts							55.4	136.8	52.9	155.5
Valuation reserve				22.6
Contract settlement, start date			Nov. 01, 2011
Contract settlement, end date			Feb. 01, 2014
Mark-to-market loss						46.4
Initial margin to open accounts for mark-to-market losses														6.9
Initial margin on deposit transferred to court-designated replacement broker															5.7
Commodity contracts value			53.3
Loss contingency, value			1.2
Outstanding debt			7.3
Outstanding debt subject to interest rate fluctuations			500.0
Notional amount of interest rate swap agreements	900.0	500.0
Number of counterparties in swap agreement		4
Interest rate swap agreements term, years		11Y
Fixed interest rate under swap agreement		5.40%
Floating interest amount, average interest addition		0.78%
Face amount of notes	1,000.0				250.0
Interest rate on debt											5.25%	5.45%
Debt instrument maturity date													Jun. 01, 2025
Effective interest rate of debt instrument											5.67%	5.68%
Number of counterparties in swap agreement settled	6
Payment for settlement of interest rate swap	35.5
Forward starting interest rate swap settlement amount			10.5
Percentage of interest in Millennium													47.50%
Unrealized loss on terminated interest rate swaps			19.2
Loss recognized in income statement classified in accumulated other comprehensive income(loss)			0.2
Contingent collateral requirement			1.9
Cash on deposit with brokers for margin requirements			81.4	158.2
Write Off Of Price Risk Asset Related With Wind Down Of Unregulated Natural Gas Business			43.8
Accounts and Notes Receivable, Net																$ 21.3