Business developments - Share transactions (Details 2) - CHF (SFr) SFr in Millions	12 Months Ended
Dec. 31, 2019	Dec. 31, 2017
Subsidiary, Sale of Stock [Line Items]
Issuances of common shares	SFr 0	SFr 0

The cash inflow from the additional capital contribution to the entity.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 14
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=118932676&loc=d3e3255-108585

Name:	us-gaap_ProceedsFromIssuanceOfCommonStock
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration