Debt - Additional Information (Details) - USD ($)
 1 Months Ended 6 Months Ended
Mar. 31, 2018
Jun. 30, 2018
Dec. 31, 2017
Jun. 30, 2017
Jul. 31, 2015
Line of Credit Facility [Line Items]          
Long-term debt, gross   $ 2,224,704,000 $ 1,967,627,000    
Revolving Credit Facility [Member]          
Line of Credit Facility [Line Items]          
Maximum borrowing capacity   $ 440,000,000      
Debt term   5 years      
Remaining borrowing capacity   $ 200,100,000      
Letters of credit issued   38,900,000 47,500,000    
Convertible debt [Member]          
Line of Credit Facility [Line Items]          
Principal of convertible senior notes   $ 225,000,000 225,000,000   $ 225,000,000
Contractual Interest Rate       4.00% 4.00%
Tsugaru Construction Loan [Member]          
Line of Credit Facility [Line Items]          
Contractual Interest Rate   0.72%      
Long-term debt, gross   $ 53,837,000 0    
Tsugaru Construction Loan [Member] | Tsugaru Credit Facility [Member]          
Line of Credit Facility [Line Items]          
Maximum borrowing capacity   $ 371,400,000.0      
Debt term, following conversion 18 years        
Tsugaru Construction Loan [Member] | Tokyo Interbank Offered Rate (TIBOR) [Member]          
Line of Credit Facility [Line Items]          
Basis spread on variable rate 0.65%        
Tsugaru Letter Of Credit Facilities [Member] | Tsugaru Credit Facility [Member]          
Line of Credit Facility [Line Items]          
Unused capacity, commitment fee percentage 0.30%        
Contractual Interest Rate   1.10%      
Debt Service Reserve Account LC [Member] | Tsugaru Credit Facility [Member]          
Line of Credit Facility [Line Items]          
Principal of convertible senior notes   $ 19,700,000      
O&M Reserve Account LC [Member] | Tsugaru Credit Facility [Member]          
Line of Credit Facility [Line Items]          
Principal of convertible senior notes   8,000,000      
Japanese Consumption Tax Facility [Member]          
Line of Credit Facility [Line Items]          
Commitment fee percentage 0.30%        
Japanese Consumption Tax Facility [Member] | Tsugaru Credit Facility [Member]          
Line of Credit Facility [Line Items]          
Maximum borrowing capacity   33,800,000.0      
Japanese Consumption Tax Facility [Member] | Tokyo Interbank Offered Rate (TIBOR) [Member] | Tsugaru Credit Facility [Member]          
Line of Credit Facility [Line Items]          
Basis spread on variable rate 0.30%        
Tsugaru Holdings Loan [Member]          
Line of Credit Facility [Line Items]          
Maximum borrowing capacity   $ 70,100,000.0      
Commitment fee percentage 0.50%        
Contractual Interest Rate   3.09%      
Long-term debt, gross   $ 58,320,000 $ 0    
Tsugaru Holdings Loan [Member] | Tokyo Interbank Offered Rate (TIBOR) [Member]          
Line of Credit Facility [Line Items]          
Basis spread on variable rate 3.00%