Additional Balance Sheet & Operational Information (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Inventories
Gas in storage – at LIFO cost	$ 36.0	$ 37.0
Coal & oil for electric generation - at average cost	43.1	42.6
Materials & supplies	46.2	48.9
Other	1.3	1.4
Total inventories	126.6	129.9
Amount by which cost of replacing inventories carried at LIFO cost exceeded carrying value	(2.0)	(1.0)
Prepayments and other current assets
Prepaid gas delivery service	26.6	26.4
Prepaid taxes	3.8	8.2
Other prepayments & current assets	16.6	18.1
Total prepayments & other current assets	47.0	52.7
Investments in unconsolidated affiliates
Investments in unconsolidated affiliates	19.7	20.4
Other utility & corporate investments
Cash surrender value of life insurance policies	42.2	33.1
Restricted cash & other investments	1.5	1.0
Total other utility & corporate investments	43.7	34.1
Goodwill	293.5	293.5
Accrued liabilities
Refunds to customers & customer deposits	51.4	49.4
Accrued taxes	55.7	46.5
Accrued interest	19.6	18.2
Deferred compensation & post retirement benefits	6.4	6.6
Accrued salaries & other	89.2	87.0
Total accrued liabilities	222.3	207.7
Asset Retirement Obligation
Asset retirement obligation, beginning balance	106.7	82.0
Accretion	4.3	3.8
Changes in estimates, net of cash payments	(4.0)	20.9
Asset retirement obligation, ending balance	107.0	106.7	$ 82.0
Equity in earnings (loss) of unconsolidated affiliates
Equity in earnings (losses) of unconsolidated affiliates	(1.1)	(0.2)	(0.6)
Other - income (expense) - net
AFUDC – borrowed funds	24.8	20.3	16.3
AFUDC – equity funds	2.6	2.2	2.6
Nonutility plant capitalized interest	1.2	1.0	0.4
Interest income, net	1.0	1.3	1.3
Other nonutility investment impairment charges	0.0	0.0	(0.1)
All other income	3.2	3.9	(0.2)
Total other income – net	32.8	28.7	20.3
Cash paid (received) for:
Interest	86.4	86.6	84.2
Income taxes	9.6	(3.6)	4.8
Accruals related to utility and nonutility plant purchases
Accruals related to utility and nonutility plant purchases	28.6	30.0
Utility Group | Gas Utility Services
Other utility & corporate investments
Goodwill	205.0	205.0
Nonutility Group | Infrastructure Services
Other utility & corporate investments
Goodwill	58.8	58.8
Nonutility Group | Energy Services
Other utility & corporate investments
Goodwill	29.7	29.7
ProLiance Holdings, LLC
Investments in unconsolidated affiliates
Investments in unconsolidated affiliates	18.9	19.2
Equity in earnings (loss) of unconsolidated affiliates
Equity in earnings (losses) of unconsolidated affiliates	(0.3)	(0.5)	(0.8)
Other nonutility partnerships & corporations
Investments in unconsolidated affiliates
Investments in unconsolidated affiliates	0.6	1.0
Other utility investments
Investments in unconsolidated affiliates
Investments in unconsolidated affiliates	0.2	0.2
Other
Equity in earnings (loss) of unconsolidated affiliates
Equity in earnings (losses) of unconsolidated affiliates	$ (0.8)	$ 0.3	$ 0.2