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Accounts Receivable (Details) - USD ($)
 3 Months Ended 12 Months Ended
Mar. 31, 2019
Mar. 31, 2018
Dec. 31, 2018
Receivables [Abstract]      
Authorized amount of receivables sold under RPAs $ 725,000,000.0    
Accounts receivable sold under RPAs 2,000,000,000.0 $ 1,900,000,000  
Cash proceeds from sale of accounts receivables 2,000,000,000.0 1,900,000,000  
Fees and financing costs under the RPA 5,800,000 $ 4,000,000.0  
Account receivable sold under the RPAs $ 479,000,000.0   $ 508,200,000