Fair value measurements Fair Value Measurements (narrative) (Details)	6 Months Ended
Jun. 30, 2019
Minimum [Member]
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Notice Period For Redemption For Alternative Investments Investment Funds	5 days
Maximum [Member]
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Notice Period For Redemption For Alternative Investments Investment Funds	120 days

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

No definition available.

Name:	us-gaap_FairValueInvestmentsEntitiesThatCalculateNetAssetValuePerShareLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration