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Debt Commitments - Industrial Revenue Bonds (Details)
Mar. 31, 2019
USD ($)
bond
Dec. 31, 2018
USD ($)
Debt Instrument [Line Items]    
Purchase of outstanding IRBs, other assets $ 98,400,000 $ 80,500,000
Debt obligation of outstanding IRBs 489,000,000 500,000,000
Industrial Revenue Bonds    
Debt Instrument [Line Items]    
Purchase of outstanding IRBs, other assets $ 19,000,000  
Industrial Revenue Bonds    
Debt Instrument [Line Items]    
Number of debt instruments | bond 2  
Debt issuance, aggregate principal amount of IRBs $ 19,000,000  
Percent of outstanding IRBs purchased 100.00%  
Debt obligation of outstanding IRBs $ 19,000,000 $ 0