Share Capital - Additional Information (Narrative) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018	Dec. 31, 2017	May 05, 2017	Sep. 15, 2015
Class of Stock [Line Items]
Amounts pledged	$ 390,832		$ 517,951
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	12,923,517	13,141,103	12,941,809	13,139,203	13,178,571
Common Stock, Shares, Issued	12,923,517		12,941,809
Repurchase and cancelled stock	79,383	0
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]
Treasury Stock, Retired, Amount Per Share	$ 25.64
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	193,401		272,784		690,734
Common Stock, Shares, Issued					13,178,571
Repurchase and cancelled stock	79,383
Treasury Stock, Retired, Cost Method, Amount	$ 2,000