Share Capital - Additional Information (Narrative) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018	Jul. 26, 2019	Mar. 31, 2019	Jan. 01, 2019	Dec. 31, 2018	Mar. 31, 2018	Dec. 31, 2017	May 05, 2017
Class of Stock [Line Items]
Amounts pledged	$ 547,326		$ 547,326					$ 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	12,756,308	13,156,353	12,756,308	13,156,353		12,923,517		12,941,809	13,141,103	13,139,203
Common Stock, Shares, Issued	12,756,308		12,756,308					12,941,809
Repurchase and cancelled stock	167,209	0	246,592	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]
Treasury Stock, Retired, Amount Per Share	$ 25.82		$ 25.76
Repurchase of class A common stock											650,000
Stock Repurchase Program, Remaining Number of Shares Authorized to be Repurchased	26,192		26,192				272,784
Repurchase and cancelled stock	167,209		246,592
Treasury Stock, Retired, Cost Method, Amount	$ 4,300		$ 6,400
Subsequent Event [Member] | 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					666,192