Consolidated Statements of Cash Flows - USD ($)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Operating Activities
Net income (loss)	$ 38,690,000	$ 102,616,000	$ (31,708,000)
Adjustments to reconcile net income (loss) to net cash provided by operating activities:
Depletion, depreciation and accretion (Note 5)	225,033,000	197,867,000	131,335,000
Asset impairment (Note 5)	0	0	1,514,000
Deferred tax expense (Note 10)	40,227,000	4,968,000	44,716,000
Stock-based compensation (Note 7)	1,430,000	8,299,000	9,775,000
Amortization of debt issuance costs (Note 6)	3,376,000	3,183,000	2,415,000
Non-cash lease expenses	1,806,000
Lease payments	(1,969,000)
Cash settlement of restricted share units	0	(360,000)	(564,000)
Unrealized foreign exchange loss	1,803,000	11,511,000	837,000
Financial instruments (gain) loss (Note 13)	(46,215,000)	12,296,000	15,929,000
Cash settlement of financial instruments	(3,273,000)	(33,931,000)	1,563,000
Cash settlement of asset retirement obligation (Note 8)	(870,000)	(519,000)	(1,336,000)
Loss on sale (Note 5)	0	0	44,385,000
Loss on redemption of Convertible Notes (Note 6)	11,501,000	0	0
Net change in assets and liabilities from operating activities (Note 14)	(93,874,000)	(21,421,000)	(29,217,000)
Net cash provided by operating activities	177,665,000	284,509,000	189,644,000
Investing Activities
Additions to property, plant and equipment (Note 5)	(379,314,000)	(347,093,000)	(251,041,000)
Property acquisitions (Note 5)	(77,772,000)	(53,200,000)	(34,410,000)
Net proceeds from sale of business units	0	0	32,968,000
Cash paid for investments	0	0	(11,000,000)
Changes in non-cash investing working capital	(7,851,000)	17,704,000	19,680,000
Net cash used in investing activities	(464,937,000)	(382,589,000)	(243,803,000)
Financing Activities
Proceeds from issuance of Senior Notes, net of issuance costs (Note 6)	289,271,000	288,131,000	0
Proceeds from bank debt, net of issuance costs	342,575,000	4,560,000	167,043,000
Repayment of debt	(349,219,000)	(153,000,000)	(110,000,000)
Proceeds from exercise of stock options	0	1,429,000	0
Repurchase of shares of Common Stock (Note 7)	(37,561,000)	(12,742,000)	(17,916,000)
Net cash provided by financing activities	245,066,000	128,378,000	39,127,000
Foreign exchange loss on cash, cash equivalents and restricted cash and cash equivalents	(1,027,000)	(2,668,000)	(1,557,000)
Net (decrease) increase in cash, cash equivalents and restricted cash and cash equivalents	(43,233,000)	27,630,000	(16,589,000)
Cash and cash equivalents and restricted cash and cash equivalents, beginning of year (Note 14)	54,308,000	26,678,000	43,267,000
Cash and cash equivalents and restricted cash and cash equivalents, end of year (Note 14)	$ 11,075,000	$ 54,308,000	$ 26,678,000