Significant Accounting Policies Property plant equipment (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Property, Plant and Equipment [Line Items]
Allowance for funds used during construction, rate	7.20%	7.50%	8.00%
Interest costs, capitalized during period	$ 7.0	$ 13.6	$ 10.8
Property, plant and equipment, disclosure of composite depreciation rate for plant in service	3.00%	3.30%	2.90%