Long-term debt (Tables)

12 Months Ended

Dec. 31, 2017

Statement [Line Items]

Disclosure of borrowings [Table Text Block]

		Dec. 31, 2017			Dec. 31, 2016
Senior unsecured notes (a)	$	987,903		$	986,574
Equipment finance facility (b)		-			50,267
Senior secured revolving credit facilities (c)		-			202,075
Less: Unamortized transaction costs - revolving credit facilities (d)		(8,328	)		(6,752	)
		979,575			1,232,164
Less: current portion		-			(16,490	)
	$	979,575		$	1,215,674

Senior secured revolving credit facilities [Member]

Statement [Line Items]

Disclosure of detailed information about borrowings [Table Text Block]

Balance, January 1, 2016	$	297,075
Addition to Principal, net of transaction costs		65,000
Payments made		(160,000	)
Balance, December 31, 2016	$	202,075
Addition to Principal		25,000
Payments made		(227,075	)
Balance, December 31, 2017	$	-

Equipment finance facility [Member]

Statement [Line Items]

Disclosure of detailed information about borrowings [Table Text Block]

Balance, January 1, 2016	$	66,521
Transaction costs		(1,013	)
Payments made		(16,490	)
Accretion of transaction costs		1,249
Balance, December 31, 2016	$	50,267
Transaction costs		(326	)
Payments made		(54,364	)
Write-down of unamortized transaction costs		3,552
Accretion of transaction costs		871
Balance, December 31, 2017	$	-

Disclosure of borrowings [Table Text Block]

		Dec. 31,		Dec. 31,
		2017		2016
Current	$	-	$	16,490
Non-current		-		33,777
	$	-	$	50,267

Senior unsecured notes [Member]

Statement [Line Items]

Disclosure of detailed information about borrowings [Table Text Block]

Balance, January 1, 2016	$	917,329
Addition to Principal, net of transaction costs		987,671
Payments made		(920,000	)
Change in fair value of embedded derivative (prepayment option)		(1,146	)
Write-down of unamortized transaction costs		2,216
Accretion of transaction costs and premiums		504
Balance, December 31, 2016	$	986,574
Transaction costs		(133	)
Change in fair value of embedded derivative (prepayment option)		450
Accretion of transaction costs and premiums		1,012
Balance, December 31, 2017	$	987,903

Unamortized transaction costs - revolving credit facilities [Member]

Statement [Line Items]

Disclosure of detailed information about unamortized transaction costs - revolving credit facilities [Table Text Block]

Balance, January 1, 2016	$	6,045
Accretion of transaction costs		(4,272	)
New transaction costs		4,979
Balance, December 31, 2016	$	6,752
Accretion of transaction costs		(3,291	)
New transaction costs		4,867
Balance, December 31, 2017	$	8,328