Subsequent events (Details) - Subsequent events	Aug. 30, 2019 USD ($) $ / shares	Aug. 30, 2023	Aug. 30, 2022	Aug. 30, 2021	Aug. 30, 2020
Term Loan | Line of Credit
Subsequent Event [Line Items]
Lender fees	$ 30,000,000.0
WageWorks, Inc
Subsequent Event [Line Items]
Merger related costs, price per share | $ / shares	$ 51.35
Consideration paid for merger	$ 2,000,000,000.0
Equity awards assumed	500,000
Cash paid	805,400,000
Borrowing pursuant to term loan facility	1,220,000,000
Credit Agreement | Line of Credit | Revolving Credit Facility
Subsequent Event [Line Items]
Credit facility, amount	$ 350,000,000
Facility term	5 years
Credit Agreement | Line of Credit | Line of Credit
Subsequent Event [Line Items]
Credit facility, amount	$ 1,600,000,000
Possible additional future loans to incur amount	$ 300,000,000
Pro forma maximum leverage ratio	3.85
Minimum interest coverage ratio	3.00
Credit Agreement | Line of Credit | Line of Credit | Debt covenant, beginning August 30, 2019
Subsequent Event [Line Items]
Maximum leverage ratio	5.25
Credit Agreement | Line of Credit | Line of Credit | Debt covenant, beginning July 31, 2020
Subsequent Event [Line Items]
Maximum leverage ratio	5.00
Credit Agreement | Line of Credit | Line of Credit | Debt covenant, beginning July 31, 2021
Subsequent Event [Line Items]
Maximum leverage ratio	4.50
Credit Agreement | Line of Credit | Line of Credit | Debt covenant, acquisition holiday provision
Subsequent Event [Line Items]
Maximum leverage ratio	5.00
Acquisition threshold for maximum total net leverage ratio	$ 100,000,000
Credit Agreement | Term Loan | Line of Credit
Subsequent Event [Line Items]
Credit facility, amount	$ 1,250,000,000
Facility term	5 years
Credit Agreement | London Interbank Offered Rate (LIBOR) | Minimum | Line of Credit | Line of Credit
Subsequent Event [Line Items]
Variable rate borrowing spread	1.25%
Credit Agreement | London Interbank Offered Rate (LIBOR) | Maximum | Line of Credit | Line of Credit
Subsequent Event [Line Items]
Variable rate borrowing spread	2.25%
Credit Agreement | Base Rate | Minimum | Line of Credit | Line of Credit
Subsequent Event [Line Items]
Variable rate borrowing spread	0.25%
Credit Agreement | Base Rate | Maximum | Line of Credit | Line of Credit
Subsequent Event [Line Items]
Variable rate borrowing spread	1.25%
Credit Agreement | Debt amortization commencing August 30, 2019 | Term Loan | Line of Credit
Subsequent Event [Line Items]
Amortization percentage of original principle amount	2.50%
Credit Agreement | Debt amortization commencing August 30, 2020 | Term Loan | Line of Credit
Subsequent Event [Line Items]
Amortization percentage of original principle amount					5.00%
Credit Agreement | Debt amortization commencing August 30, 2021 | Term Loan | Line of Credit
Subsequent Event [Line Items]
Amortization percentage of original principle amount				5.00%
Credit Agreement | Debt amortization commencing August 30, 2022 | Term Loan | Line of Credit
Subsequent Event [Line Items]
Amortization percentage of original principle amount			7.50%
Credit Agreement | Debt amortization commencing August 30, 2023 | Term Loan | Line of Credit
Subsequent Event [Line Items]
Amortization percentage of original principle amount		10.00%