Summary of significant accounting policies - Recently Adopted Accounting Pronouncements (Details) - USD ($) $ in Millions	Jan. 01, 2019	Dec. 31, 2020	Dec. 31, 2019
Recently Adopted Accounting Pronouncements
Package practical expedients	true
Hindsight practical expedient	false
Operating lease right-of-use assets		$ 163	$ 176
Operating lease liability		$ 199
ASU 2016-02
Recently Adopted Accounting Pronouncements
Operating lease right-of-use assets	$ 133
Operating lease liability	$ 176