Acquisition Additional Information (Details)
 3 Months Ended 6 Months Ended
Aug. 10, 2017
USD ($)
MW
Apr. 21, 2017
USD ($)
kV
mi
MW
Sep. 30, 2017
USD ($)
Jun. 30, 2017
USD ($)
Jun. 30, 2017
USD ($)
Jul. 27, 2017
USD ($)
Dec. 31, 2016
USD ($)
Business Acquisition [Line Items]              
Equity Method Investments     $ 303,833,000       $ 233,294,000
Long-term Debt, Gross     2,225,857,000       $ 1,605,462,000
Equity Method Investment, Transaction Costs Capitalized     1,500,000        
Cost in excess of equity     0        
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    
Decrease in Property, Plant, and Equipment     900,000        
Decrease in Accrued Construction Costs     1,200,000        
Increase in Asset Retirement Obligations     $ 300,000        
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          
Meikle [Member]              
Business Acquisition [Line Items]              
Business Acquisition, Percentage of Voting Interests Acquired 50.99%            
Generating Capacity | MW 179            
Payments to Acquire Businesses, Gross $ 67,400,000            
Current portion of long-term debt including accrued interest $ 265,600,000            
Meikle Wind Energy Limited Partnership [Member]              
Business Acquisition [Line Items]              
Business Acquisition, Percentage of Voting Interests Acquired 70.00%            
Meikle and Meikle Wind Energy Corp. [Member]              
Business Acquisition [Line Items]              
Business Combination, Consideration Transferred $ 67,400,000.0            
Business Acquisition, Transaction Costs Capitalized $ 1,100,000            
Pattern Development 2.0 [Member]              
Business Acquisition [Line Items]              
Equity Method Investments           $ 60,000,000  
Equity Method Investment, Ownership Percentage           20.00%  
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          
Public Sector Pension Investment Board [Member] | Meikle [Member]              
Business Acquisition [Line Items]              
Business Acquisition, Percentage of Voting Interests Acquired 48.99%            
Public Sector Pension Investment Board [Member] | Meikle Wind Energy Limited Partnership [Member]              
Business Acquisition [Line Items]              
Business Acquisition, Percentage of Voting Interests Acquired 30.00%            
Public Sector Pension Investment Board [Member] | Meikle and Meikle Wind Energy Corp. [Member]              
Business Acquisition [Line Items]              
Business Combination, Consideration Transferred $ 64,800,000.0