Operations and Significant Accounting Policies - Reclassification of Prior Income Statement (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Operating Revenue – Utility [Member]
Schedule of Prior Period Reclassification Adjustments [Line Items]
Prior Period Reclassification Adjustment	$ 1,063.8	$ 1,000.7
Operating Revenue – Non-utility [Member]
Schedule of Prior Period Reclassification Adjustments [Line Items]
Prior Period Reclassification Adjustment	331.9	316.7
Other Operating Revenue - Non-utility [Member]
Schedule of Prior Period Reclassification Adjustments [Line Items]
Prior Period Reclassification Adjustment	$ 23.6	$ 22.3