Interim financial statements - Recently Adopted Accounting Standard (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Jan. 01, 2019	Dec. 31, 2018
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Right-of-use assets	$ 103,947	$ 99,800	$ 0
Lease liabilities	115,307	99,800	0
Accounts payable and accrued liabilities	$ (71,621)		$ (109,283)
ASU 2016-02
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Right-of-use assets		10,700
Accounts payable and accrued liabilities		$ 10,700