Recently issued accounting standards - Recently issued accounting standards (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Summary Of Significant Accounting Policies [Line Items]
Cumulative-effect adjustment	$ (3,336,457)	$ (2,979,320)	$ (2,539,978)	$ (2,430,531)
Retained Earnings
Summary Of Significant Accounting Policies [Line Items]
Cumulative-effect adjustment	$ (3,096,228)	(2,824,916)	(2,427,599)	(2,285,886)
Cumulative Effect, Period of Adoption, Adjustment
Summary Of Significant Accounting Policies [Line Items]
Cumulative-effect adjustment		791	1,321	(3,076)
Cumulative Effect, Period of Adoption, Adjustment | Retained Earnings
Summary Of Significant Accounting Policies [Line Items]
Cumulative-effect adjustment		$ 791	$ 1,321	$ (3,076)