Share Capital - Stock Option Activity (Details) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2017 USD ($) $ / shares shares		Dec. 31, 2017 USD ($) $ / shares CAD / shares shares		Dec. 31, 2016 USD ($) $ / shares shares	Dec. 31, 2016 CAD / shares	Dec. 31, 2015 USD ($) $ / shares shares	Dec. 31, 2015 CAD / shares	Dec. 31, 2017 CAD / shares
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Number of Options Outstanding, Beginning balance	1,910,823				1,721,472		1,484,218
Number of Options, Granted	620,950				323,794		529,288
Number of Options, Exercised	(71,006)	[1]			(121,292)		(270,254)
Number of Options, Forfeited, cancelled or expired	(120,862)				(13,151)		(21,780)
Number of Options Outstanding, Ending balance	2,339,905				1,910,823		1,721,472
Number of Options Exercisable	1,439,301		1,439,301
Weighted Average Exercise Price Outstanding, Beginning balance | (per share)	$ 7.32		CAD 9.84		$ 6.95	CAD 9.62	$ 4.20	CAD 4.88
Weighted Average Exercise Price, Granted | (per share)	6.69		8.69		7.39	9.79	14.67	18.73
Weighted Average Exercise Price, Exercised | (per share)	2.86	[1]	3.72	[1]	2.77	3.67	3.65	4.66
Weighted Average Exercise Price, Forfeited, cancelled and expired | (per share)	6.98		9.06		11.31	14.98	8.78	11.21
Weighted Average Exercise Price Outstanding, Ending balance | (per share)	7.41		CAD 9.32		$ 7.32	CAD 9.84	$ 6.95	CAD 9.62
Weighted Average Exercise Price, Exercisable Ending balance | (per share)	$ 7.08		CAD 7.08						CAD 8.91
Aggregate Intrinsic Value, Outstanding, Beginning balance | $	$ 4,464				$ 1,880		$ 15,551
Aggregate Intrinsic Value, Exercised | $	108	[1]			568		2,426
Aggregate Intrinsic Value, Outstanding, Ending balance | $	159				$ 4,464		$ 1,880
Aggregate Intrinsic Value, Exercisable, Ending balance | $	$ 159		CAD 159

[1] During the year ended December 31, 2017, 63,425 (2016 – 47,413) stock options were exercised for the same number of common shares in exchange for cash. In the same period, the Company issued 4,405 (2016 – 47,841) common shares for the cashless exercise of 7,581 (2016 – 73,879) stock options.