Stock compensation plans - Share-based Compensation Activity (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Share-based Payment Arrangement [Abstract]
Stock-based compensation	$ 20.7	$ 26.9	$ 22.4
Tax benefit from compensation expense	22.0	21.1	20.7
Net excess tax benefit from compensation expense	$ 17.5	$ 15.4	$ 15.9

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

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
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-Section 50
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-Publisher FASB
-Name Accounting Standards Codification
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-Publisher FASB
-Name Accounting Standards Codification
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