Fair Value (Tables)

12 Months Ended

Dec. 31, 2017

Fair Value Disclosures [Abstract]

Schedule of Fair Value, Assets and Liabilities Measured on Recurring Basis

The following tables present financial assets and liabilities measured and recorded at fair value on NiSource’s Consolidated Balance Sheets on a recurring basis and their level within the fair value hierarchy as of December 31, 2017 and December 31, 2016:

Recurring Fair Value Measurements December 31, 2017 (in millions)	Quoted Prices in Active Markets for Identical Assets (Level 1)		Significant Other Observable Inputs (Level 2)		Significant Unobservable Inputs (Level 3)		Balance as of December 31, 2017
Assets
Risk management assets	$	—	$	21.1	$	—	$	21.1
Available-for-sale securities	—		133.9		—		133.9
Total	$	—	$	155.0	$	—	$	155.0
Liabilities
Risk management liabilities	$	—	$	71.4	$	0.3	$	71.7
Total	$	—	$	71.4	$	0.3	$	71.7

Recurring Fair Value Measurements December 31, 2016 (in millions)	Quoted Prices in Active Markets for Identical Assets (Level 1)		Significant Other Observable Inputs (Level 2)		Significant Unobservable Inputs (Level 3)		Balance as of December 31, 2016
Assets
Risk management assets	$	5.4	$	43.6	$	—	$	49.0
Available-for-sale securities	—		131.5		—		131.5
Total	$	5.4	$	175.1	$	—	$	180.5
Liabilities
Risk management liabilities	$	1.2	$	58.9	$	1.2	$	61.3
Total	$	1.2	$	58.9	$	1.2	$	61.3

Available-For-Sale Debt Securities

The amortized cost, gross unrealized gains and losses and fair value of available-for-sale securities at December 31, 2017 and 2016 were:

December 31, 2017 (in millions)	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses			Fair Value
Available-for-sale securities
U.S. Treasury debt securities	$	26.9	$	—	$	(0.1	)	$	26.8
Corporate/Other debt securities	106.8		0.9		(0.6		)	107.1
Total	$	133.7	$	0.9	$	(0.7	)	$	133.9
December 31, 2016 (in millions)	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses			Fair Value
Available-for-sale securities
U.S. Treasury debt securities	$	35.0	$	0.1	$	(0.6	)	$	34.5
Corporate/Other debt securities	98.7		0.3		(2.0		)	97.0
Total	$	133.7	$	0.4	$	(2.6	)	$	131.5

Carrying Amount And Estimated Fair Values Of Financial Instruments

The carrying amount and estimated fair values of these financial instruments were as follows:

At December 31, (in millions)	Carrying Amount 2017		Estimated Fair Value 2017		Carrying Amount 2016		Estimated Fair Value 2016
Long-term debt (including current portion)	$	7,796.5	$	8,603.4	$	6,421.3	$	7,064.1