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Capital and Reserves (Details 1) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Disclosure of share capital, reserves and other equity interest [Line Items]      
Balance $ 1,011,314 $ 971,931 $ 911,039
Beginning Balance (in Shares) 39,160,048 38,968,905 38,777,339
Repurchase of common stock $ (28) $ 0 $ 0
Restricted stock issued - directors 30 0 0
Balance $ 1,042,812 $ 1,011,314 $ 971,931
Ending Balance (in Shares) 39,428,835 39,160,048 38,968,905
Treasury stock [member]      
Disclosure of share capital, reserves and other equity interest [Line Items]      
Balance $ (69,176) $ (73,397) $ (77,627)
Beginning Balance (in Shares) 2,819,791 3,010,934 3,202,500
Repurchase of common stock $ (28) $ 0 $ 0
Repurchase of common stock (in Shares) 1,000 0 0
Restricted stock issued - directors $ 1,259 $ 1,259 $ 1,259
Restricted stock issued - directors (in Shares) (57,000) (57,000) (57,000)
Exercised stock options - compensation plans $ (3,140) $ (1,519) $ (1,553)
Exercised stock options - compensation plans (in Shares) (142,268) (68,785) (70,358)
Restricted stock units - vested $ (1,557) $ (1,443) $ (1,418)
Restricted stock units - vested (in Shares) (70,519) (65,358) (64,208)
Balance $ (63,248) $ (69,176) $ (73,397)
Ending Balance (in Shares) 2,551,004 2,819,791 3,010,934
Class A shares [Member] | Treasury stock [member]      
Disclosure of share capital, reserves and other equity interest [Line Items]      
Balance $ 10,708 $ 10,708 $ 10,708
Beginning Balance (in Shares) 318,140 318,140 318,140
Repurchase of common stock $ 0 $ 0 $ 0
Repurchase of common stock (in Shares) 0 0 0
Restricted stock issued - directors $ 0 $ 0 $ 0
Restricted stock issued - directors (in Shares) 0 0 0
Exercised stock options - compensation plans $ 0 $ 0 $ 0
Exercised stock options - compensation plans (in Shares) 0 0 0
Restricted stock units - vested $ 0 $ 0 $ 0
Restricted stock units - vested (in Shares) 0 0 0
Balance $ 10,708 $ 10,708 $ 10,708
Ending Balance (in Shares) 318,140 318,140 318,140
Class B shares [Member] | Treasury stock [member]      
Disclosure of share capital, reserves and other equity interest [Line Items]      
Balance $ 16,242 $ 16,242 $ 16,242
Beginning Balance (in Shares) 589,174 589,174 589,174
Repurchase of common stock $ 28 $ 0 $ 0
Repurchase of common stock (in Shares) 1,000 0 0
Restricted stock issued - directors $ 0 $ 0 $ 0
Restricted stock issued - directors (in Shares) 0 0 0
Exercised stock options - compensation plans $ 0 $ 0 $ 0
Exercised stock options - compensation plans (in Shares) 0 0 0
Restricted stock units - vested $ 0 $ 0 $ 0
Restricted stock units - vested (in Shares) 0 0 0
Balance $ 16,270 $ 16,242 $ 16,242
Ending Balance (in Shares) 590,174 589,174 589,174
Class E shares [Member] | Treasury stock [member]      
Disclosure of share capital, reserves and other equity interest [Line Items]      
Balance $ 42,226 $ 46,447 $ 50,677
Beginning Balance (in Shares) 1,912,477 2,103,620 2,295,186
Repurchase of common stock $ 0 $ 0 $ 0
Repurchase of common stock (in Shares) 0 0 0
Restricted stock issued - directors $ (1,259) $ (1,259) $ (1,259)
Restricted stock issued - directors (in Shares) (57,000) (57,000) (57,000)
Exercised stock options - compensation plans $ (3,140) $ (1,519) $ (1,553)
Exercised stock options - compensation plans (in Shares) (142,268) (68,785) (70,358)
Restricted stock units - vested $ (1,557) $ (1,443) $ (1,418)
Restricted stock units - vested (in Shares) (70,519) (65,358) (64,208)
Balance $ 36,270 $ 42,226 $ 46,447
Ending Balance (in Shares) 1,642,690 1,912,477 2,103,620