Other Commitments And Contingencies Other Commitments and Contingencies (Contractual Obligation, Fiscal Year Maturity Schedule) (Details) - USD ($) $ in Millions		Dec. 31, 2016	Dec. 31, 2015
Long-term Purchase Commitment [Line Items]
Long-term Debt, Future Minimum Payments Due	[1]	$ 6,305.5
Long-term Debt, Future Minimum Payments Due, Next Twelve Months	[1]	349.9
Long-term Debt, Future Minimum Payments, Due in Two Years	[1]	476.0
Long-term Debt, Future Minimum Payments, Due in Three Years	[1]	1,041.0
Long-term Debt, Future Minimum Payments, Due in Four Years	[1]	550.0
Long-term Debt, Future Minimum Payments, Due in Five Years	[1]	63.6
Long-term Debt, Future Minimum Payments, Due Thereafter	[1]	3,825.0
Capital Leases, Future Minimum Payments Due	[2]	250.0
Capital Leases, Future Minimum Payments Due, Next Twelve Months	[2]	22.7
Capital Leases, Future Minimum Payments Due in Two Years	[2]	18.5
Capital Leases, Future Minimum Payments Due in Three Years	[2]	14.2
Capital Leases, Future Minimum Payments Due in Four Years	[2]	13.5
Capital Leases, Future Minimum Payments Due in Five Years	[2]	13.4
Capital Leases, Future Minimum Payments Due Thereafter	[2]	167.7
Interest Payments on Long-term Debt, Future Minimum Payments Due		4,611.2
Interest Payments on Long-term Debt, Future Minimum Payments Due, Next Twelve Months		337.9
Interest Payments on Long-term Debt, Future Minimum Payments, Due in Two Years		305.3
Interest Payments on Long-term Debt, Future Minimum Payments, Due in Three Years		265.2
Interest Payments on Long-term Debt, Future Minimum Payments, Due in Four Years		244.9
Interest Payments on Long-term Debt, Future Minimum Payments, Due in Five Years		214.9
Interest Payments on Long-term Debt, Future Minimum Payments, Due Thereafter		3,243.0
Operating Leases, Future Minimum Payments Due	[3]	54.6
Operating Leases, Future Minimum Payments Due, Next Twelve Months	[3]	15.4
Operating Leases, Future Minimum Payments, Due in Two Years	[3]	9.4
Operating Leases, Future Minimum Payments, Due in Three Years	[3]	7.5
Operating Leases, Future Minimum Payments, Due in Four Years	[3]	4.8
Operating Leases, Future Minimum Payments, Due in Five Years	[3]	4.1
Operating Leases, Future Minimum Payments, Due Thereafter	[3]	13.4
Energy Commodity Contracts, Future Minimum Payments Due	[4]	312.1
Energy Commodity Contracts, Future Minimum Payments Due, Next Twelve Months	[4]	108.5
Energy Commodity Contracts, Future Minimum Payments, Due in Two Years	[4]	67.7
Energy Commodity Contracts, Future Minimum Payments, Due in Three Years	[4]	67.3
Energy Commodity Contracts, Future Minimum Payments, Due in Four Years	[4]	68.0
Energy Commodity Contracts, Future Minimum Payments, Due in Five Years	[4]	0.6
Energy Commodity Contracts, Future Minimum Payments, Due Thereafter	[4]	0.0
Service Obligations, Pipeline Service Obligations, Future Minimum Payments Due		2,002.1
Service Obligations, Pipeline Service Obligations, Future Minimum Payments Due, Next Twelve Months		532.7
Service Obligations, Pipeline Service Obligations, Future Minimum Payments, Due in Two Years		382.7
Service Obligations, Pipeline Service Obligations, Future Minimum Payments, Due in Three Years		293.1
Service Obligations, Pipeline Service Obligations, Future Minimum Payments, Due in Four Years		176.0
Service Obligations, Pipeline Service Obligations, Future Minimum Payments, Due in Five Years		139.2
Service Obligations, Pipeline Service Obligations, Future Minimum Payments, Due Thereafter		478.4
Service Obligations, IBM Service Obligations, Future Minimum Payments Due		325.0
Service Obligations, IBM Service Obligations, Future Minimum Payments Due, Next Twelve Months		84.1
Service Obligations, IBM Service Obligations, Future Minimum Payments, Due in Two Years		81.2
Service Obligations, IBM Service Obligations, Future Minimum Payments, Due in Three Years		80.0
Service Obligations, IBM Service Obligations, Future Minimum Payments, Due in Four Years		79.7
Service Obligations, IBM Service Obligations, Future Minimum Payments, Due in Five Years		0.0
Service Obligations, IBM Service Obligations, Future Minimum Payments, Due Thereafter		0.0
Service Obligations, Other Service Obligations, Future Minimum Payments Due		77.7
Service Obligations, Other Service Obligations, Future Minimum Payments Due, Next Twelve Months		58.1
Service Obligations, Other Service Obligations, Future Minimum Payments, Due in Two Years		17.4
Service Obligations, Other Service Obligations, Future Minimum Payments, Due in Three Years		1.9
Service Obligations, Other Service Obligations, Future Minimum Payments, Due in Four Years		0.3
Service Obligations, Other Service Obligations, Future Minimum Payments, Due in Five Years		0.0
Service Obligations, Other Service Obligations, Future Minimum Payments, Due Thereafter		0.0
Other Liabilities, Future Minimum Payments Due		34.4
Other Liabilities, Future Minimum Payments Due, Next Twelve Months		34.4
Other Liabilities, Future Minimum Payments, Due in Two Years		0.0
Other Liabilities, Future Minimum Payments, Due in Three Years		0.0
Other Liabilities, Future Minimum Payments, Due in Four Years		0.0
Other Liabilities, Future Minimum Payments, Due in Five Years		0.0
Other Liabilities, Future Minimum Payments, Due Thereafter		0.0
Total Future Minimum Lease Payments Due		13,972.6
Total Future Minimum Lease Payments Due, Next Twelve Months		1,543.7
Total Future Minimum Lease Payments, Due in Two Years		1,358.2
Total Future Minimum Lease Payments, Due in Three Years		1,770.2
Total Future Minimum Lease Payments, Due in Four Years		1,137.2
Total Future Minimum Lease Payments, Due in Five Years		435.8
Total Future Minimum Lease Payments, Due Thereafter		7,727.5
Debt Instrument, Unamortized Discount (Premium) and Debt Issuance Costs, Net		(41.6)	$ (45.5)
Capital Leases, Future Minimum Payments, Interest Included in Payments		92.6
Fleet Lease
Long-term Purchase Commitment [Line Items]
Operating Leases, Future Minimum Payments Due, Next Twelve Months		31.1
Operating Leases, Future Minimum Payments, Due in Two Years		32.9
Operating Leases, Future Minimum Payments, Due in Three Years		26.1
Operating Leases, Future Minimum Payments, Due in Four Years		17.5
Operating Leases, Future Minimum Payments, Due in Five Years		8.0
Operating Leases, Future Minimum Payments, Due Thereafter		2.0
NIPSCO Coal Contracts
Long-term Purchase Commitment [Line Items]
Energy Commodity Contracts, Future Minimum Payments Due, Next Twelve Months		24.2
Energy Commodity Contracts, Future Minimum Payments, Due in Two Years		$ 10.1

[1] Long-term debt balance excludes unamortized issuance costs and discounts of $41.6 million.

[2] Capital lease payments shown above are inclusive of interest totaling $92.6 million.

[3] Operating lease balances do not include amounts for fleet leases that can be renewed beyond the initial lease term. The Company anticipates renewing the leases beyond the initial term, but the anticipated payments associated with the renewals do not meet the definition of expected minimum lease payments and therefore are not included above. Expected payments are $31.1 million in 2017, $32.9 million in 2018, $26.1 million in 2019, $17.5 million in 2020, $8.0 million in 2021 and $2.0 million thereafter.

[4] )In January 2017, NIPSCO signed new coal contract commitments of $24.2 million and $10.1 million for 2017 and 2018, respectively. These contracts are not included above.