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, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Class of Stock [Line Items]
Class A Stock outstanding, beginning of period (in shares)	13,132,775	13,259,715	13,261,095	13,238,486
Issued pursuant to shared-based compensation plans (in shares)	19,883	10,988	198,903	283,471
Repurchased and canceled pursuant to the stock buy-back (in shares)	(143,010)	(2,264)	(450,350)	(253,518)
Class A Stock outstanding, end of period (in shares)	13,009,648	13,268,439	13,009,648	13,268,439