Stock-based compensation - other information (Details) - USD ($) shares in Millions, $ in Millions	3 Months Ended
Mar. 31, 2021	Mar. 31, 2020
Other information about options outstanding
Remaining expense to be recognized	$ 114.8
Weighted average period over which remaining expense will be recognized	1 year 10 months 24 days
Shares available for future issuance	9.8
Expense	$ 11.4	$ 8.4
Common Stock Including Additional Paid in Capital
Other information about options outstanding
Options exercised in the period (in shares)	1.0
Stock options
Other information about options outstanding
Expense	$ 4.6	3.9
Contractual life	10 years
RSUs
Other information about options outstanding
Expense	$ 5.7	$ 4.9
Minimum \| RSUs
Other information about options outstanding
Award vesting period	3 years
Maximum \| Stock options
Other information about options outstanding
Award vesting period	4 years
Maximum \| RSUs
Other information about options outstanding
Award vesting period	4 years

Amount of expense for award under share-based payment arrangement. Excludes amount capitalized.

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Period over which grantee's right to exercise award under share-based payment arrangement is no longer contingent on satisfaction of service or performance condition, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days. Includes, but is not limited to, combination of market, performance or service condition.

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