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Postretirement Benefit Obligations - Narrative (Details) - USD ($)
$ in Millions
1 Months Ended 3 Months Ended 12 Months Ended
Apr. 30, 2018
Mar. 31, 2018
Dec. 31, 2018
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Pension contributions   $ 2.0  
Subsequent Event      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Pension contributions $ 1.6   $ 8.0