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Commitments and Contingencies, Leases (Details)
$ in Millions
12 Months Ended
Dec. 31, 2016
USD ($)
Lease
Dec. 31, 2015
USD ($)
Dec. 31, 2014
USD ($)
Capital Leases [Abstract]      
Number of leases qualifying as capital leases | Lease 3    
Amortization expense under capital lease assets $ 8.1 $ 8.2 $ 7.2
Property Held Under Capital Leases, Net [Abstract]      
Property held under capital lease 221.2 221.2  
Accumulated depreciation (65.3) (57.2)  
Total property held under capital leases, net 155.9 164.0  
Capital Leases, Future Minimum Payments Due [Abstract]      
2017 25.7    
2018 25.3    
2019 25.1    
2020 24.9    
2021 23.8    
Thereafter 462.7    
Total minimum obligation 587.5    
Interest component of obligation (431.6)    
Present value of minimum obligation $ 155.9    
Operating Leased Assets [Line Items]      
Operating Lease Purchase Power Agreement Expiration Date 2032    
Operating Leases, Future Minimum Payments Due [Abstract]      
2017 $ 107.3    
2018 106.8    
2019 107.8    
2020 108.7    
2021 109.1    
Thereafter $ 528.9    
WYCO Totem Gas Storage Facilities      
Capital Leases [Abstract]      
Ownership interest in joint venture (in hundredths) 50.00%    
Capital lease obligations $ 127.0 132.9  
Gas Storage Facilities      
Property Held Under Capital Leases, Net [Abstract]      
Property held under capital lease 200.5 200.5  
Gas Pipeline      
Property Held Under Capital Leases, Net [Abstract]      
Property held under capital lease 20.7 20.7  
Office Space and Other Equipment      
Operating Leases [Abstract]      
Total expenses under operating lease obligations 118.2 130.5 126.2
Operating Leases, Future Minimum Payments Due [Abstract]      
2017 11.0    
2018 10.2    
2019 10.3    
2020 10.3    
2021 9.7    
Thereafter 45.2    
Purchased Power Agreements      
Operating Leases [Abstract]      
Payments for capacity for PPAs under operating lease obligations 102.4 $ 113.5 $ 110.1
Operating Leases, Future Minimum Payments Due [Abstract]      
2017 [1],[2] 96.3    
2018 [1],[2] 96.6    
2019 [1],[2] 97.5    
2020 [1],[2] 98.4    
2021 [1],[2] 99.4    
Thereafter [1],[2] $ 483.7    
[1] Amounts do not include PPAs accounted for as executory contracts.
[2] PPA operating leases contractually expire through 2032.