Debt and letter of credit facilities - Summary of letter of credit facilities (Details) - Standby letters of credit	Jun. 30, 2023 USD ($)
Line of Credit Facility [Line Items]
Issued	$ 1,297,000,000
Cash and Cash Equivalents
Line of Credit Facility [Line Items]
Collateral	31,900,000
Debt securities
Line of Credit Facility [Line Items]
Collateral	1,379,000,000
Committed - Secured letters of credit facilities
Line of Credit Facility [Line Items]
Committed Capacity	395,000,000.0
Issued	310,000,000.0
Committed - Secured letters of credit facilities | Cash and Cash Equivalents
Line of Credit Facility [Line Items]
Collateral	17,600,000
Committed - Secured letters of credit facilities | Debt securities
Line of Credit Facility [Line Items]
Collateral	214,700,000
Uncommitted - Secured letters of credit facilities
Line of Credit Facility [Line Items]
Issued	987,000,000.0
Uncommitted - Secured letters of credit facilities | Cash and Cash Equivalents
Line of Credit Facility [Line Items]
Collateral	14,300,000
Uncommitted - Secured letters of credit facilities | Debt securities
Line of Credit Facility [Line Items]
Collateral	$ 1,164,300,000