Pledged securities owned and pledged as collateral amount eligible to be repledged by counterparty carrying value.

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The maximum number of common shares permitted to be issued by an entity's charter and bylaws.

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The number of shares authorized to be repurchased by an entity's Board of Directors under a stock repurchase plan.

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Number of shares that have been repurchased and retired during the period.

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Share Capital - Additional Information (Narrative) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020	Jun. 30, 2021	Dec. 31, 2020	Jun. 30, 2020	May 15, 2020	Dec. 31, 2019
Class of Stock [Line Items]
Amounts pledged	$ 424,103		$ 424,103			$ 440,531
Preferred stock, authorized	50,000,000		50,000,000
Preferred stock, par value	$ 0.001		$ 0.001
Preferred stock, issued	0		0
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	12,515,734	12,404,427	12,515,734	12,404,427	12,592,646	12,381,778	12,445,479	12,636,523	12,698,703
Common Stock, Shares, Issued	12,515,734		12,515,734			12,381,778
Repurchase and cancelled stock	108,494	84,290	108,494	684,838
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 Repurchased and Retired During Period, Value	$ 4,700	$ 2,000		$ 14,200
Stock Repurchase Program, Remaining Number of Shares Authorized to be Repurchased	292,519		292,519					628,625
Stock Repurchased and Retired During Period, Shares	108,494	84,290		684,838
Stock Repurchased and Retired During Period, Per Share	$ 43.46	$ 23.28		$ 20.78
Stock Repurchased and Retired During Period, Shares	108,494	84,290		684,838