Share Capital - Changes in Number of Shares of Class A Stock Outstanding (Details) - Class A Stock [Member] - shares	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Class of Stock [Line Items]
Class A Stock outstanding, beginning of period (in shares)	13,182,332	13,263,532	13,261,095	13,238,486
Issued pursuant to shared-based compensation plans (in shares)	0	7,163	179,020	272,483
Repurchased and canceled pursuant to the stock buy-back (in shares)	(49,557)	(10,980)	(307,340)	(251,254)
Class A Stock outstanding, end of period (in shares)	13,132,775	13,259,715	13,132,775	13,259,715