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Share Capital - Changes in Number of Shares of Class A Stock Outstanding (Details) - Class A Stock [Member] - shares
 3 Months Ended 9 Months Ended
Sep. 30, 2015
Sep. 30, 2014
Sep. 30, 2015
Sep. 30, 2014
Class of Stock [Line Items]        
Class A Stock outstanding, beginning of period (in shares) 13,650,149 13,519,126 13,530,688 13,377,967
Issued pursuant to shared-based compensation plans (in shares) 12,063 11,562 131,524 152,721
Repurchased and canceled pursuant to the stock buy-back (in shares) (328,844) 0 (328,844) 0
Class A Stock outstanding, end of period (in shares) 13,333,368 13,530,688 13,333,368 13,530,688