Share Capital - Additional Information (Narrative) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017	May 05, 2017	Dec. 31, 2016	Sep. 15, 2015
Class of Stock [Line Items]
Amounts pledged	$ 688,647		$ 655,683
Preferred stock, authorized	50,000,000
Preferred stock, par value	$ 0.001
Preferred stock, issued	0
Class A Stock
Class of Stock [Line Items]
Common stock, authorized	50,000,000		50,000,000
Common stock, par value	$ 0.001		$ 0.001
Common Stock, outstanding	13,141,103	13,182,332	13,139,203		13,261,095	13,348,369
Common Stock, Shares, Issued	13,141,103		13,139,203
Repurchase and cancelled stock	0	257,783
Class B Stock
Class of Stock [Line Items]
Common stock, authorized	99,665		99,665
Common stock, par value	$ 0.001		$ 0.001
Common Stock, outstanding	99,665		99,665
Common Stock, Shares, Issued	99,665		99,665
Previous Program [Member] | Class A Stock
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
New Program [Member] | Class A Stock
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 Repurchase Program, Remaining Number of Shares Authorized to be Repurchased	508,906			690,734
Common Stock, Shares, Issued				13,178,571