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Stock-Based Compensation - 2017 Retention Program Narrative (Details) - Restricted Stock Awards - USD ($)
$ in Millions
1 Months Ended 12 Months Ended
Jul. 31, 2017
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Granted (in shares) 379,472 437,472 750 1,000 23,000
Outstanding $ 2.9        
Cash awards $ 3.3