Summary of significant accounting policies - Accounting pronouncements (Details) - USD ($)	Jan. 01, 2019	Dec. 31, 2017
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Change in accounting principle		$ 948,000
Minimum | Restatement Adjustment | Subsequent Event | Accounting Standards Update 2016-02
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Right-of-use assets	$ 45,000,000
Lease liabilities	45,000,000
Maximum | Restatement Adjustment | Subsequent Event | Accounting Standards Update 2016-02
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Right-of-use assets	65,000,000
Lease liabilities	$ 65,000,000
Retained earnings
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Change in accounting principle		948,000
Retained earnings | Accounting Standards Update 2014-09
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Change in accounting principle		$ 948,000