Segment Information (Details) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2016 USD ($)	Sep. 30, 2015 USD ($)	Sep. 30, 2016 USD ($) segment	Sep. 30, 2015 USD ($)
Segment Reporting Information
Number of reportable segments | segment			2
Operating revenues	$ 1,023.8	$ 981.1	$ 2,876.5	$ 2,948.7
Other operation and maintenance	359.4	355.3	1,043.8	1,040.9
Depreciation and amortization	81.9	76.2	243.1	226.5
Operating income	196.4	169.8	524.8	503.2
Equity in earnings of transmission affiliate	14.6	15.7	40.7	42.1
Interest expense	29.5	30.4	$ 88.0	88.6
Utility
Segment Reporting Information
Number of service areas within the natural gas service area			3
Operating revenues	1,023.8	981.1	$ 2,876.5	2,948.7
Other operation and maintenance	359.4	355.3	1,043.8	1,040.9
Depreciation and amortization	81.9	76.2	243.1	226.5
Operating income	196.4	169.8	524.8	503.2
Equity in earnings of transmission affiliate	0.0	0.0	0.0	0.0
Interest expense	29.3	30.1	87.3	87.6
Other
Segment Reporting Information
Operating revenues	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
Operating income	0.0	0.0	0.0	0.0
Equity in earnings of transmission affiliate	14.6	15.7	40.7	42.1
Interest expense	$ 0.2	0.3	$ 0.7	1.0
ATC
Segment Reporting Information
Ownership interest in ATC (as a percent)	23.00%		23.00%
Equity in earnings of transmission affiliate	$ 14.6	$ 15.7	$ 40.7	$ 42.1