Pension And Other Postretirement Benefits (Components Of The Plans' Net Periodic Benefits Cost) (Details) - USD ($) $ in Millions		3 Months Ended
		Mar. 31, 2017	Mar. 31, 2016
Pension Plan
Defined Benefit Plan Disclosure [Line Items]
Service Cost	[1]	$ 7.5	$ 7.7
Interest cost	[1]	17.2	22.4
Expected return on assets		(30.3)	(33.2)
Amortization of prior service credit		(0.2)	(0.1)
Recognized actuarial loss		13.4	15.3
Total Net Periodic Benefits Cost		7.6	12.1
Other Postretirement Benefit Plan
Defined Benefit Plan Disclosure [Line Items]
Service Cost	[1]	1.2	1.2
Interest cost	[1]	4.5	5.5
Expected return on assets		(4.0)	(4.3)
Amortization of prior service credit		(1.1)	(1.2)
Recognized actuarial loss		0.8	0.8
Total Net Periodic Benefits Cost		$ 1.4	$ 2.0

[1] Effective January 1, 2017, NiSource adopted the methodology of using a full yield curve (spot rate) approach to estimate the service and interest components of net periodic benefit cost. This change in accounting estimate resulted in a decrease in these costs for the three months ended March 31, 2017 when compared to the same period in 2016.