Common Shares, Redeemable Common Shares and Preferred Shares Common shares by class (Details) - shares	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Class of Stock [Line Items]
Shares, Outstanding	22,880,652	24,220,859	24,923,140
Treasury Stock, Shares, Acquired	(2,636,943)	(1,697,727)
Shares sold	960,740	662,806
Stock subscription offering, employee cash purchases	129,245	90,645
Options exercised	206,751	241,995
Permanent equity [Member]
Class of Stock [Line Items]
Shares, Outstanding	17,238,497	17,753,832	17,866,236
Treasury Stock, Shares, Acquired	(1,530,528)	(807,718)
Shares sold	679,197	362,674
Stock subscription offering, employee cash purchases	129,245	90,645
Options exercised	206,751	241,995
Temporary equity [Member]
Class of Stock [Line Items]
Shares, Outstanding	5,642,155	6,467,027	7,056,904
Treasury Stock, Shares, Acquired	(1,106,415)	(890,009)
Shares sold	281,543	300,132
Stock subscription offering, employee cash purchases	0	0
Options exercised	0	0