Share Capital - Additional Information (Narrative) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Jun. 30, 2018	Dec. 31, 2017	Jun. 30, 2017	May 05, 2017	Dec. 31, 2016	Sep. 15, 2015
Class of Stock [Line Items]
Amounts pledged	$ 738,835		$ 738,835			$ 655,683
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	13,177,931	13,009,648	13,177,931	13,009,648	13,156,353	13,139,203	13,132,775	13,178,571	13,261,095
Common Stock, Shares, Issued	13,177,931		13,177,931			13,139,203
Repurchase and cancelled stock	0	143,010	0	450,350
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]
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		508,906					690,734
Common Stock, Shares, Issued								13,178,571