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Employee Benefits - Amounts Not Yet Recognized in Net Periodic Benefit Cost (Details) - USD ($)
$ in Millions
Dec. 31, 2017
Dec. 31, 2016
Pension Costs    
Accumulated other comprehensive loss (pre-tax)    
Net actuarial loss (gain) $ 10.0 $ 12.0
Prior service credits 0.0 0.0
Total 10.0 12.0
Net regulatory assets    
Net actuarial loss (gain) 1,136.8 1,240.7
Prior service costs (credits) 7.5 10.5
Total 1,144.3 1,251.2
Estimated amounts that will be amortized into net periodic benefit cost next year    
Net actuarial loss 92.5  
Prior service costs (credits) 2.6  
Total 2018 – estimated amortization 95.1  
OPEB Costs    
Accumulated other comprehensive loss (pre-tax)    
Net actuarial loss (gain) (1.0) (1.0)
Prior service credits (0.1) 0.0
Total (1.1) (1.0)
Net regulatory assets    
Net actuarial loss (gain) (4.7) 25.8
Prior service costs (credits) (111.8) (87.9)
Total (116.5) $ (62.1)
Estimated amounts that will be amortized into net periodic benefit cost next year    
Net actuarial loss 1.3  
Prior service costs (credits) (15.3)  
Total 2018 – estimated amortization $ (14.0)