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Postretirement Benefit Obligations - Narrative (Details) - USD ($)
$ in Millions
1 Months Ended 6 Months Ended
Jul. 31, 2017
Apr. 30, 2017
Jan. 31, 2017
Jun. 30, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Pension contributions   $ 1.6 $ 2.2 $ 17.0
Subsequent Event        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Pension contributions $ 1.9