Operations and Significant Accounting Policies - Reclassification of Prior Income Statement (Details) $ in Millions	3 Months Ended
	Mar. 31, 2017 USD ($)
Operating Revenue to Operating Revenue – Utility [Member]
Reclassification of Prior Income Statement [Line Items]
Prior Period Reclassification Amount	$ 281.6
Operating Revenue – Non-utility [Member]
Reclassification of Prior Income Statement [Line Items]
Prior Period Reclassification Amount	84.0
Cost of Sales to Fuel, Purchased Power and Gas – Utility [Member]
Reclassification of Prior Income Statement [Line Items]
Prior Period Reclassification Amount	$ 3.6