Share Capital - Changes in Number of Shares of Class A Stock Outstanding (Details) - Class A Stock - shares	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward]
Class A Stock outstanding, beginning of period	13,141,103	13,182,332	13,139,203	13,261,095
Issued pursuant to shared-based compensation plans	15,250	0	17,150	179,020
Repurchased and canceled pursuant to the stock buy-back	0	(49,557)	0	(307,340)
Class A Stock outstanding, end of period	13,156,353	13,132,775	13,156,353	13,132,775