Recently issued accounting standards Recently issued accounting standards (Details) - USD ($) $ in Thousands	Jan. 01, 2019	Dec. 31, 2018	Jan. 01, 2018	Dec. 31, 2017
Summary Of Significant Accounting Policies [Line Items]
Retained earnings		$ 2,427,599		$ 2,285,886
Reserve for returns and allowances		$ 4,200		$ 0
ASU 2014-09 | Difference between Revenue Guidance in Effect before and after Topic 606
Summary Of Significant Accounting Policies [Line Items]
Retained earnings			$ 1,200
ASU 2016-02 | Forecast | Maximum
Summary Of Significant Accounting Policies [Line Items]
Operating lease, right-of-use asset	$ 110,000
ASU 2016-16
Summary Of Significant Accounting Policies [Line Items]
New accounting pronouncement or change in accounting principle, cumulative effect of change on equity or net assets			$ 1,800
Subsequent Event | ASU 2016-02 | Forecast | Minimum
Summary Of Significant Accounting Policies [Line Items]
Operating lease, right-of-use asset	90,000
Lease liabilities	90,000
Subsequent Event | ASU 2016-02 | Forecast | Maximum
Summary Of Significant Accounting Policies [Line Items]
Lease liabilities	$ 110,000