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Stock Compensation - Restricted Stock and Performance Share Grants - Compensation Costs (Details) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Stock compensation costs $ 4,881 $ 3,922 $ 3,629
1998 Plan      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Stock compensation costs, expensed 3,847 2,989 2,645
Stock compensation costs, capitalized 296 165 95
Stock compensation costs 4,143 3,154 2,740
NDSI Plan      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Stock compensation costs, expensed $ 738 $ 768 $ 889