Regulatory Assets and Liabilities - Regulatory Assets (Details) - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015
Regulatory Assets
Current assets	$ 50.4	$ 37.1
Regulatory assets	3,087.9	3,064.6
Total Regulatory Assets	3,138.3	3,101.7
Other Disclosures
Regulatory assets not earning a return	32.7
Regulatory assets earnings return based on short-term interest rates	204.0
Environmental remediation liabilities	633.6	628.2
Unrecognized pension and OPEB costs
Regulatory Assets
Total Regulatory Assets	1,252.1	1,306.4
Environmental remediation costs
Regulatory Assets
Total Regulatory Assets	702.7	697.0
Other Disclosures
Environmental remediation liabilities	633.6
Income tax related items
Regulatory Assets
Total Regulatory Assets	285.1	248.3
Electric transmission costs
Regulatory Assets
Total Regulatory Assets	234.1	191.5
SSR
Regulatory Assets
Total Regulatory Assets	188.1	86.1
AROs
Regulatory Assets
Total Regulatory Assets	179.2	173.0
We Power generation
Regulatory Assets
Total Regulatory Assets	54.1	45.4
Energy efficiency programs
Regulatory Assets
Total Regulatory Assets	36.7	48.7
Derivatives
Regulatory Assets
Total Regulatory Assets	17.9	70.4
Other, net
Regulatory Assets
Total Regulatory Assets	$ 188.3	$ 234.9