Long-term debt (Tables)

12 Months Ended

Dec. 31, 2018

Statements [Line Items]

Disclosure of borrowings [Table Text Block]

		Dec. 31, 2018			Dec. 31, 2017			Jan. 1, 2017
Senior unsecured notes (a)	$	989,306		$	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)		(8,276	)		(8,328	)		(6,752	)
		981,030			979,575			1,232,164
Less: current portion		-			-			(16,490	)
	$	981,030		$	979,575		$	1,215,674

Senior secured revolving credit facilities [Member]

Statements [Line Items]

Disclosure of detailed information about borrowings [Table Text Block]

Balance, January 1, 2017	$	202,075
Addition to Principal		25,000
Payments made		(227,075	)
Balance, December 31, 2017	$	—

Equipment finance facility [Member]

Statements [Line Items]

Disclosure of detailed information about borrowings [Table Text Block]

Balance, January 1, 2017	$	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	$	—

Senior unsecured notes [Member]

Statements [Line Items]

Disclosure of detailed information about borrowings [Table Text Block]

Balance, January 1, 2017	$	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
Change in fair value of embedded derivative (prepayment option)		316
Accretion of transaction costs and premiums		1,087
Balance, December 31, 2018	$	989,306

Unamortized transaction costs - revolving credit facilities [Member]

Statements [Line Items]

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

Balance, January 1, 2017	$	6,752
Accretion of transaction costs		(3,291	)
Transaction costs		4,867
Balance, December 31, 2017	$	8,328
Accretion of transaction costs		(1,946	)
Transaction costs		1,894
Balance, December 31, 2018	$	8,276