Operations and Significant Accounting Policies - New Accounting Pronouncements (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2018
New Accounting Standards [Line Items]
Cumulative Effect of New Accounting Principle in Period of Adoption	$ 0.5
Accounting Standards Update 2018-02 [Member]
New Accounting Standards [Line Items]
Cumulative Effect of New Accounting Principle in Period of Adoption			$ 5.7
Accounting Standards Update 2017-07 [Member] | Operating and Maintenance [Member]
New Accounting Standards [Line Items]
Prior Period Reclassification Adjustment	4.2	$ 6.8
Accounting Standards Update 2017-07 [Member] | Cost of Sales - Non-utility [Member]
New Accounting Standards [Line Items]
Prior Period Reclassification Adjustment	0.3
Accounting Standards Update 2017-07 [Member] | Other Income/Expense [Member]
New Accounting Standards [Line Items]
Prior Period Reclassification Adjustment	3.9	6.5
Accounting Standards Update 2016-01 [Member]
New Accounting Standards [Line Items]
Cumulative Effect of New Accounting Principle in Period of Adoption	0.1
Accounting Standards Update 2016-15 [Member]
New Accounting Standards [Line Items]
Prior Period Reclassification Adjustment		2.9
Accounting Standards Update 2014-09 [Member]
New Accounting Standards [Line Items]
Cumulative Effect of New Accounting Principle in Period of Adoption	0.5
Accounting Standards Update 2016-02 [Member]
New Accounting Standards [Line Items]
Expected Minimum Lease Payment Due in Future Years			$ 36.0
Financing Activities [Member] | Accounting Standards Update 2016-18 [Member]
New Accounting Standards [Line Items]
Prior Period Reclassification Adjustment	$ 0.4	0.7
Investing Activities [Member] | Accounting Standards Update 2016-18 [Member]
New Accounting Standards [Line Items]
Prior Period Reclassification Adjustment		$ 4.1