Share Capital - Additional Information (Narrative) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Jun. 30, 2020	May 15, 2020	Dec. 31, 2019	Jun. 30, 2019	Dec. 31, 2018
Class of Stock [Line Items]
Amounts pledged	$ 486,165		$ 486,165				$ 357,120
Preferred stock, authorized	50,000,000		50,000,000
Preferred stock, par value	$ 0.001		$ 0.001
Preferred stock, issued	0		0
Stock Repurchased and Retired During Period, Value	$ 2,000		$ 14,200
Treasury Stock Acquired, Average Cost Per Share	$ 23.28		$ 20.78
Stock Repurchased and Retired During Period, Shares	84,290		684,838
Class A Stock
Class of Stock [Line Items]
Common stock, authorized	50,000,000		50,000,000				50,000,000
Common stock, par value	$ 0.001		$ 0.001				$ 0.001
Common Stock, outstanding	12,404,427	12,698,703	12,404,427	12,698,703	12,445,479	12,636,523	12,698,703	12,756,308	12,941,809
Common Stock, Shares, Issued	12,404,427		12,404,427				12,698,703
Repurchase and cancelled stock	84,290	76,657	684,838	323,249
Class B Stock
Class of Stock [Line Items]
Common stock, authorized	99,665		99,665				99,665
Common stock, par value	$ 0.001		$ 0.001				$ 0.001
Common Stock, outstanding	99,665		99,665				99,665
Common Stock, Shares, Issued	99,665		99,665				99,665
Previous Program [Member] | Class A Stock
Class of Stock [Line Items]
Stock Repurchase Program, Remaining Number of Shares Authorized to be Repurchased						98,625
New Program [Member] | Class A Stock
Class of Stock [Line Items]
Repurchase of class A common stock						530,000
Stock Repurchase Program Percentage Of Shares Repurchase Of Outstanding Share						4.20%
Stock Repurchase Program, Remaining Number of Shares Authorized to be Repurchased	434,697		434,697			628,625