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Stock and Other Incentive Plans (Details) - Stock Incentive Plan [Member] - USD ($)
$ / shares in Units, $ in Thousands
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Summary of the activity under the incentive plans      
Stock-based awards, beginning of year (shares) 1,786,497 1,092,262 1,057,732
Granted (shares) 413,489 885,219 251,789
Vested (shares) (292,341) (190,984) (210,955)
Forfeited (shares) 0 0 (6,304)
Stock-based awards, end of year (shares) 1,907,645 1,786,497 1,092,262
Weighted-average grant date fair value of:      
Stock-based awards, beginning of year (in USD per share) $ 27.18 $ 24.72 $ 24.01
Stock-based awards granted during the year (in USD per share) 32.05 29.60 27.70
Stock-based awards vested during the year (in USD per share) 25.88 24.34 25.05
Stock-based awards forfeited during the year (in USD per share) 0.00 0.00 24.80
Stock-based awards, end of year (in USD per share) $ 28.44 $ 27.18 $ 24.72
Grant date fair value of shares granted during the year $ 13,254 $ 26,204 $ 6,975