Summary of significant accounting policies - Recently Issued Accounting Standards (Details) $ in Millions	12 Months Ended
	Dec. 31, 2019 USD ($) Period	Jan. 01, 2019 USD ($)	Dec. 31, 2018 USD ($)	Jan. 01, 2018 USD ($)	Dec. 31, 2017 USD ($)
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Operating lease, liability	$ 25.7
Adoption of new accounting standard (ASC 606), net of tax				$ (32.5)
Contract liability	$ 88.9		$ 73.1		$ 30.5
CIADM
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Base period	7 years
Term of contract	24 years
Number of additional option periods | Period	17
Accounting Standards Update 2016-02
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Operating lease right-of-use assets		$ 13.4
Operating lease, liability		$ 14.0
Accounting Standards Update 2014-09
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Contract liability				72.9
Difference between Revenue Guidance in Effect before and after Topic 606 | Accounting Standards Update 2014-09
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Contract liability				42.4
Difference between Revenue Guidance in Effect before and after Topic 606 | Accounting Standards Update 2014-09 | CIADM
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Contract liability				42.4
Deferred tax assets				9.9
Retained Earnings
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Adoption of new accounting standard (ASC 606), net of tax				(32.5)
Retained Earnings | Accounting Standards Update 2014-09
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Adoption of new accounting standard (ASC 606), net of tax				32.5
Retained Earnings | Accounting Standards Update 2014-09 | CIADM
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Adoption of new accounting standard (ASC 606), net of tax				$ 32.5