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

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Percentage of repurchase of class A common stock.

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Face amount or stated value per share of common stock.

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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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The remaining 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								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	May 15, 2020
Class of Stock [Line Items]
Amounts pledged	$ 440,531				$ 357,120				$ 440,531	$ 357,120
Preferred stock, authorized	50,000,000								50,000,000
Preferred stock, par value	$ 0.001								$ 0.001
Preferred stock, issued	0								0
Dividends	$ 1.12	$ 0.12	$ 0.12	$ 0.12	$ 0.12	$ 0.12	$ 0.11	$ 0.11	$ 1.48	$ 0.46
Class A Stock
Class of Stock [Line Items]
Common stock, authorized	50,000,000				50,000,000				50,000,000	50,000,000
Common stock, par value	$ 0.001				$ 0.001				$ 0.001	$ 0.001
Common Stock, Shares, Issued	12,381,778				12,698,703				12,381,778	12,698,703
Repurchase and cancelled stock									718,522	323,249
Dividends									$ 1.48	$ 0.46	$ 0.44
Special Cash Dividend \| Class A Stock
Class of Stock [Line Items]
Dividends, Cash									$ 12,500
Dividends									$ 1.00
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, Shares									718,522	323,249
Stock Repurchased and Retired During Period, Value									$ 15,000	$ 8,400
Stock Repurchased and Retired During Period, Per Share									$ 20.94	$ 25.99
Stock Repurchase Program, Remaining Number of Shares Authorized to be Repurchased	401,013				589,535				401,013	589,535		628,625