Report of Business Segments (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015	Dec. 31, 2015
Segment Reporting Information [Line Items]
Operating revenues	$ 551.4	$ 549.9	$ 984.5	$ 1,030.0
Cost of sales	197.7	210.9	375.6	422.5
Other operation and maintenance	127.6	113.2	241.5	224.9
Depreciation and amortization	80.1	76.2	158.6	152.1
Taxes other than income	20.1	22.4	45.0	46.9
OPERATING INCOME	125.9	127.2	163.8	183.6
Equity in earnings of unconsolidated affiliates	16.7	28.2	45.0	59.9
Other income (expense)	5.5	5.1	11.0	10.5
Interest expense	36.0	38.0	72.3	75.4
Income tax expense (benefit)	40.6	35.0	50.8	47.9
Net income (loss)	71.5	87.5	96.7	130.7
Investment in unconsolidated affiliates	1,168.8	1,309.2	1,168.8	1,309.2	$ 1,194.4
Total assets	9,659.2	9,576.1	9,659.2	9,576.1	$ 9,580.6
Electric Utility [Member]
Segment Reporting Information [Line Items]
Operating revenues	551.4	549.9	984.5	1,030.0
Cost of sales	197.7	210.9	375.6	422.5
Other operation and maintenance	124.8	115.6	241.1	229.9
Depreciation and amortization	78.4	74.3	155.1	148.1
Taxes other than income	19.1	21.6	42.7	44.7
OPERATING INCOME	131.4	127.5	170.0	184.8
Equity in earnings of unconsolidated affiliates	0.0	0.0	0.0	0.0
Other income (expense)	7.0	4.4	12.3	7.3
Interest expense	35.0	37.3	70.5	74.1
Income tax expense (benefit)	31.1	25.6	33.4	31.9
Net income (loss)	72.3	69.0	78.4	86.1
Investment in unconsolidated affiliates	0.0	0.0	0.0	0.0
Total assets	8,380.2	8,315.7	8,380.2	8,315.7
Natural Gas Midstream Operations [Member]
Segment Reporting Information [Line Items]
Operating revenues	0.0	0.0	0.0	0.0
Cost of sales	0.0	0.0	0.0	0.0
Other operation and maintenance	7.8	0.2	8.0	1.0
Depreciation and amortization	0.0	0.0	0.0	0.0
Taxes other than income	0.0	0.0	0.0	0.0
OPERATING INCOME	(7.8)	(0.2)	(8.0)	(1.0)
Equity in earnings of unconsolidated affiliates	16.7	28.2	45.0	59.9
Other income (expense)	0.0	0.0	0.0	0.0
Interest expense	0.0	0.0	0.0	0.0
Income tax expense (benefit)	9.3	10.0	19.4	18.1
Net income (loss)	(0.4)	18.0	17.6	40.8
Investment in unconsolidated affiliates	1,168.8	1,309.2	1,168.8	1,309.2
Total assets	1,481.8	1,486.6	1,481.8	1,486.6
Other Operations [Member]
Segment Reporting Information [Line Items]
Operating revenues	0.0	0.0	0.0	0.0
Cost of sales	0.0	0.0	0.0	0.0
Other operation and maintenance	(5.0)	(2.6)	(7.6)	(6.0)
Depreciation and amortization	1.7	1.9	3.5	4.0
Taxes other than income	1.0	0.8	2.3	2.2
OPERATING INCOME	2.3	(0.1)	1.8	(0.2)
Equity in earnings of unconsolidated affiliates	0.0	0.0	0.0	0.0
Other income (expense)	(1.4)	0.8	(1.1)	3.3
Interest expense	1.1	0.8	2.0	1.4
Income tax expense (benefit)	0.2	(0.6)	(2.0)	(2.1)
Net income (loss)	(0.4)	0.5	0.7	3.8
Investment in unconsolidated affiliates	0.0	0.0	0.0	0.0
Total assets	94.9	121.8	94.9	121.8
Eliminations [Member]
Segment Reporting Information [Line Items]
Operating revenues	0.0	0.0	0.0	0.0
Cost of sales	0.0	0.0	0.0	0.0
Other operation and maintenance	0.0	0.0	0.0	0.0
Depreciation and amortization	0.0	0.0	0.0	0.0
Taxes other than income	0.0	0.0	0.0	0.0
OPERATING INCOME	0.0	0.0	0.0	0.0
Equity in earnings of unconsolidated affiliates	0.0	0.0	0.0	0.0
Other income (expense)	(0.1)	(0.1)	(0.2)	(0.1)
Interest expense	(0.1)	(0.1)	(0.2)	(0.1)
Income tax expense (benefit)	0.0	0.0	0.0	0.0
Net income (loss)	0.0	0.0	0.0	0.0
Investment in unconsolidated affiliates	0.0	0.0	0.0	0.0
Total assets	$ (297.7)	$ (348.0)	$ (297.7)	$ (348.0)