Interim financial statements - Recently Adopted Accounting Standard (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Jan. 01, 2019	Dec. 31, 2018
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Right-of-use assets	$ 100,671	$ 99,800
Lease liabilities	112,260	99,800
Accounts payable and accrued liabilities	$ 103,886		$ 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