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Capital Stock and Share Based Compensation Plans - Additional restricted stock and restricted stock unit vesting (Details) - Restricted Stock Units (RSUs) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Fair value of RSUs vested $ 12,090 $ 7,898 $ 15,697
Tax benefits realized on shares and units vested [1] $ 4,630 $ 3,033 $ 5,965
[1] (1) Benefits for 2015 were not recognized under prior accounting guidance until realized. In 2017 and 2016, upon adoption of new accounting guidance, they are realized when generated.