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Summary of Significant Accounting Policies - Investments in Affiliates and Other Entities (Details)
 3 Months Ended 12 Months Ended
Mar. 31, 2019
USD ($)
Dec. 31, 2018
USD ($)
Sep. 30, 2019
USD ($)
Dec. 31, 2014
substation
kV
km
Summary Of Significant Accounting Policies [Line Items]        
Contributions to partnership in connection with certain investments and management fees     $ 13,700,000  
Water and Gas Pipeline Infrastructure Contractor        
Summary Of Significant Accounting Policies [Line Items]        
Equity interest acquired   30.00%    
Cash subscription price   $ 22,200,000    
Cash dividends   $ 3,900,000    
Equity Method Investment in Electric Power Infrastructure Services Company        
Summary Of Significant Accounting Policies [Line Items]        
Equity interest   49.00%    
Payments to acquire equity method investments and intangible assets   $ 12,300,000    
EPC Electric Transmission Project        
Summary Of Significant Accounting Policies [Line Items]        
Capital commitment     0  
Length of electrical transmission line to be constructed under contract | km       500
Number of substations | substation       2
Voltage of substations | kV       500
Deferred earnings recognized $ 60,300,000      
Capital for Infrastructure Projects        
Summary Of Significant Accounting Policies [Line Items]        
Capital commitment     80,000,000.0  
Infrastructure Investors Partnership | Capital for Infrastructure Projects        
Summary Of Significant Accounting Policies [Line Items]        
Capital commitment     $ 1,000,000,000.0