Common Shares and Earnings Per Common Share - Additional Information (Details) - $ / shares	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Number of common shares authorized to repurchase	12,500,000
3.875% Convertible Senior Debentures, due November 1, 2029
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Convertible notes interest rate	3.875%	3.875%	3.875%
Warrants
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Securities excluded from computation of earnings per share amount		923,077	4,000,000
Exercise price (USD per share)		$ 33.33	$ 33.33
Stock options
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Securities excluded from computation of earnings per share amount		656,894	1,000,137
Redeemable Convertible Preferred Shares
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Securities excluded from computation of earnings per share amount	4,162,200	4,162,200	4,162,200