Summary of significant accounting policies - Narratives (Details) - USD ($) $ in Millions	Mar. 31, 2019	Jan. 01, 2019
Leases
Operating lease right-of-use assets	$ 129
Operating lease liability	$ 170
ASU 2016-02
Leases
Operating lease right-of-use assets		$ 133
Operating lease liability		$ 176

Present value of lessee's discounted obligation for lease payments from operating lease.

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 842
-SubTopic 20
-Section 45
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=77888419&loc=SL77918627-209977

Name:	us-gaap_OperatingLeaseLiability
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant