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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Stock Repurchased and Retired During Period, Per Share

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Aggregate dividends paid during the period for each share of common stock outstanding.

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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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Total number of common shares of an entity that have been sold or granted to shareholders (includes common shares that were issued, repurchased and remain in the treasury). These shares represent capital invested by the firm's shareholders and owners, and may be all or only a portion of the number of shares authorized. Shares issued include shares outstanding and shares held in the treasury.

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Number of shares of common stock outstanding. Common stock represent the ownership interest in a corporation.

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Face amount or stated value per share of preferred stock nonredeemable or redeemable solely at the option of the issuer.

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The maximum number of nonredeemable preferred shares (or preferred stock redeemable solely at the option of the issuer) permitted to be issued by an entity's charter and bylaws.

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Total number of nonredeemable preferred shares (or preferred stock redeemable solely at the option of the issuer) issued to shareholders (includes related preferred shares that were issued, repurchased, and remain in the treasury). May be all or portion of the number of preferred shares authorized. Excludes preferred shares that are classified as debt.

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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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Equity impact of the value of stock that has been repurchased and retired during the period. The excess of the purchase price over par value can be charged against retained earnings (once the excess is fully allocated to additional paid in capital).

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Number of shares of common and preferred stock retired from treasury 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, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	May 05, 2017	Sep. 15, 2015
Class of Stock [Line Items]
Amounts pledged	$ 655,683				$ 438,385				$ 655,683	$ 438,385
Preferred stock, authorized	50,000,000								50,000,000
Preferred stock, par value	$ 0.001								$ 0.001
Preferred stock, issued	0								0
Dividends per share	$ 0.11	$ 0.11	$ 0.11	$ 0.11	$ 0.11	$ 0.11	$ 0.11	$ 0.11	$ 0.44000	$ 0.44	$ 440.00
Common Class A [Member]
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, outstanding	13,139,203				13,261,095				13,139,203	13,261,095	13,238,486		13,348,369
Common Stock, Shares, Issued	13,139,203				13,261,095				13,139,203	13,261,095
Repurchase and cancelled stock									450,350	260,862
Dividends per share									$ 0.44	$ 0.44	$ 0.44
Class B Stock
Class of Stock [Line Items]
Common stock, authorized	99,665				99,665				99,665	99,665
Common stock, par value	$ 0.001				$ 0.001				$ 0.001	$ 0.001
Common Stock, outstanding	99,665				99,665				99,665	99,665
Common Stock, Shares, Issued	99,665				99,665				99,665	99,665
Dividends per share									$ 0.44	$ 0.44	$ 0.44
Previous Program [Member] \| Common Class A [Member]
Class of Stock [Line Items]
Repurchase of class A common stock													665,000
Stock Repurchase Program, Remaining Number of Shares Authorized to be Repurchased	40,734								40,734
New Program [Member] \| Common Class A [Member]
Class of Stock [Line Items]
Repurchase of class A common stock												650,000
Stock Repurchase Program Percentage Of Shares Repurchase Of Outstanding Share												5.00%
Stock Repurchased and Retired During Period, Shares									450,350
Stock Repurchased and Retired During Period, Value									$ 7,500
Stock Repurchased and Retired During Period, Per Share									$ 16.57
Stock Repurchase Program, Remaining Number of Shares Authorized to be Repurchased	508,906								508,906			690,734
Common Stock, Shares, Issued												13,178,571