Quarterly report pursuant to Section 13 or 15(d)

Summary of Significant Accounting Policies - Investments in Affiliates and Other Entities (Details)

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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 ($)
Dec. 31, 2018
USD ($)
Dec. 31, 2014
substation
kV
km
Summary Of Significant Accounting Policies [Line Items]      
Contributions to partnership in connection with certain investments and management fees $ 15,100,000    
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