Share Capital - Additional Information (Narrative) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Jul. 26, 2019	Jun. 30, 2019	Dec. 31, 2018	Jun. 30, 2018	Dec. 31, 2017
Class of Stock [Line Items]
Amounts pledged	$ 359,151		$ 359,151				$ 517,951
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	12,698,703	13,177,931	12,698,703	13,177,931	12,756,308	12,756,308	12,941,809	13,156,353	13,139,203
Common Stock, Shares, Issued	12,698,703		12,698,703				12,941,809
Repurchase and cancelled stock	76,657	0	323,249	0
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						26,192
New Program [Member] | Class A Stock
Class of Stock [Line Items]
Repurchase of class A common stock					640,000
Stock Repurchase Program Percentage Of Shares Repurchase Of Outstanding Share					5.00%
Stock Repurchase Program, Remaining Number of Shares Authorized to be Repurchased	589,535		589,535		666,192
Repurchase and cancelled stock	76,657		323,249
Treasury Stock, Retired, Cost Method, Amount	$ 2,000		$ 8,400
Treasury Stock Acquired, Average Cost Per Share	$ 26.71		$ 25.99