Segment Reporting - Narrative (Details) customer in Millions, $ in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 2017 segment	Dec. 31, 2017 USD ($) customer group	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Segment Reporting Information [Line Items]
Number of operating groups | group		3
Portion of Indiana that is provided natural gas distribution and transportation services by the Gas Utility Services segment (in hundredths)		66.67%
Number of customers | customer		1
Utility Group
Segment Reporting Information [Line Items]
Number of operating segments | segment	3
Fees incurred for pipeline construction and repair services | $		$ 157.1	$ 117.8	$ 109.5