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Acquisition Additional Information (Details)
3 Months Ended 6 Months Ended
Apr. 21, 2017
USD ($)
kV
mi
MW
Jun. 30, 2017
USD ($)
Jun. 30, 2017
USD ($)
Dec. 31, 2016
USD ($)
Business Acquisition [Line Items]        
Long-term Debt, Gross   $ 1,872,120,000 $ 1,872,120,000 $ 1,605,462,000
Class B membership interest in Broadview Holdings [Member]        
Business Acquisition [Line Items]        
Ownership Percentage 100.00%      
Western Interconnect [Member]        
Business Acquisition [Line Items]        
Ownership Percentage 99.00%      
Broadview Holdings' Interest in Broadview [Member]        
Business Acquisition [Line Items]        
Ownership Percentage 100.00%      
Initial distributable cash flow from Broadview [Member]        
Business Acquisition [Line Items]        
Ownership Percentage 84.00%      
Noncontrolling Interest, Ownership Percentage by Noncontrolling Owners 16.00%      
Class A membership interest in Broadview Holdings by NCI [Member]        
Business Acquisition [Line Items]        
Ownership Percentage 100.00%      
Interest in Western Interconnect by third party [Member]        
Business Acquisition [Line Items]        
Ownership Percentage 1.00%      
Broadview Project [Member]        
Business Acquisition [Line Items]        
Business Acquisition, Percentage of Voting Interests Acquired 100.00%      
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Contingent Liability $ 36,205,000      
Payments to Acquire Businesses, Gross 214,700,000      
Business Combination, Consideration Transferred, Liabilities Incurred 2,400,000      
Business Combination, Contingent Consideration, Liability 21,300,000      
Current portion of long-term debt including accrued interest 51,200,000      
Business Acquisition, Transaction Costs   $ 400,000 $ 400,000  
Broadview Project [Member] | Continued Operation Of Broadview [Member]        
Business Acquisition [Line Items]        
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Contingent Liability 7,200,000      
Broadview Project [Member] | Continued Operation Of Grady [Member]        
Business Acquisition [Line Items]        
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Contingent Liability $ 29,000,000      
Western Interconnect [Member]        
Business Acquisition [Line Items]        
Transmission line length | mi 35      
Transmission-level voltage | kV 345      
Long-term Debt, Gross $ 54,400,000.0      
Broadview [Member]        
Business Acquisition [Line Items]        
Generating Capacity | MW 324