Fair Value Disclosures (Fair Value Of Financial Assets And Liabilities Measured On A Recurring Basis) (Details) (USD $) In Millions, unless otherwise specified	Jun. 30, 2012	Dec. 31, 2011
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Available-for-sale securities	$ 100.0	$ 96.0
Fair value assets measured on recurring basis, total	310.3	444.5
Fair value liabilities measured on recurring basis, total	178.3	306.7
Physical Price Risk Programs [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Fair value assets measured on recurring basis, derivative financial instruments	55.9	140.7
Fair value liabilities measured on recurring basis, derivative financial instruments	0.5	3.9
Financial Price Risk Programs [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Fair value assets measured on recurring basis, derivative financial instruments	105.9	151.1
Fair value liabilities measured on recurring basis, derivative financial instruments	177.8	302.8
Interest Rate Risk Activities [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Fair value assets measured on recurring basis, derivative financial instruments	48.5	56.7
Quoted Prices In Active Markets For Identical Assets (Level 1) [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Available-for-sale securities	30.7	32.9
Fair value assets measured on recurring basis, total	135.1	181.2
Fair value liabilities measured on recurring basis, total	176.5	301.1
Quoted Prices In Active Markets For Identical Assets (Level 1) [Member] | Physical Price Risk Programs [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Fair value assets measured on recurring basis, derivative financial instruments	0	0
Fair value liabilities measured on recurring basis, derivative financial instruments	0	0
Quoted Prices In Active Markets For Identical Assets (Level 1) [Member] | Financial Price Risk Programs [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Fair value assets measured on recurring basis, derivative financial instruments	104.4	148.3	[1]
Fair value liabilities measured on recurring basis, derivative financial instruments	176.5	301.1
Quoted Prices In Active Markets For Identical Assets (Level 1) [Member] | Interest Rate Risk Activities [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Fair value assets measured on recurring basis, derivative financial instruments	0	0
Significant Other Observable Inputs (Level 2) [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Available-for-sale securities	69.3	63.1
Fair value assets measured on recurring basis, total	175.2	263.0
Fair value liabilities measured on recurring basis, total	1.5	5.6
Significant Other Observable Inputs (Level 2) [Member] | Physical Price Risk Programs [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Fair value assets measured on recurring basis, derivative financial instruments	55.9	140.7
Fair value liabilities measured on recurring basis, derivative financial instruments	0.5	3.9
Significant Other Observable Inputs (Level 2) [Member] | Financial Price Risk Programs [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Fair value assets measured on recurring basis, derivative financial instruments	1.5	2.5
Fair value liabilities measured on recurring basis, derivative financial instruments	1.0	1.7
Significant Other Observable Inputs (Level 2) [Member] | Interest Rate Risk Activities [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Fair value assets measured on recurring basis, derivative financial instruments	48.5	56.7
Significant Unobservable Inputs (Level 3) [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Available-for-sale securities	0	0
Fair value assets measured on recurring basis, total	0	0.3
Fair value liabilities measured on recurring basis, total	0.3	0
Significant Unobservable Inputs (Level 3) [Member] | Physical Price Risk Programs [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Fair value assets measured on recurring basis, derivative financial instruments	0	0
Fair value liabilities measured on recurring basis, derivative financial instruments	0	0
Significant Unobservable Inputs (Level 3) [Member] | Financial Price Risk Programs [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Fair value assets measured on recurring basis, derivative financial instruments	0	0.3
Fair value liabilities measured on recurring basis, derivative financial instruments	0.3	0
Significant Unobservable Inputs (Level 3) [Member] | Interest Rate Risk Activities [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Fair value assets measured on recurring basis, derivative financial instruments	$ 0	$ 0

[1] During the fourth quarter of 2011, NiSource recorded a reserve of $22.6 million on certain assets related to the wind down of the unregulated natural gas marketing business. The financial price risk program amount above is shown gross and has not been adjusted for the reserve.