Regulatory Assets and Liabilities - Regulatory Assets (Details) - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015
Regulatory Assets
Total regulatory assets	$ 2,036.6	$ 1,855.9
Other Disclosures
Regulatory assets not earning a return	10.4
Regulatory assets earning a return based on short-term interest rates	204.0
Plant related -- capital leases
Regulatory Assets
Total regulatory assets	724.8	674.4
Unrecognized pension and OPEB costs
Regulatory Assets
Total regulatory assets	520.3	535.8
Electric transmission costs
Regulatory Assets
Total regulatory assets	231.9	191.5
Income tax related items
Regulatory Assets
Total regulatory assets	200.8	177.4
SSR
Regulatory Assets
Total regulatory assets	188.1	86.1
We Power generation
Regulatory Assets
Total regulatory assets	54.1	45.4
AROs
Regulatory Assets
Total regulatory assets	39.7	36.3
Energy efficiency programs
Regulatory Assets
Total regulatory assets	38.5	50.7
Other, net
Regulatory Assets
Total regulatory assets	$ 38.4	$ 58.3