Summary of significant accounting policies - Recent adopted changes in accounting principle and Recent accounting pronouncements (Details) - USD ($) $ in Millions	6 Months Ended
	Jun. 30, 2020	Mar. 31, 2020	Jan. 01, 2020	Dec. 31, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018
Summary of significant accounting policies
Allowance for credit loss	$ 0.4
Cumulative effect of an accounting change	$ 1,522.2			$ 1,642.8	$ 1,787.1
Accounting Standards Update [Extensible List]	us-gaap:AccountingStandardsUpdate201613Member
Accounting Standards Update 2016-13
Summary of significant accounting policies
Allowance for credit loss			$ 14.9
Retained earnings
Summary of significant accounting policies
Cumulative effect of an accounting change	$ 660.0	$ 673.7		778.5	$ 918.5	$ 911.8	$ 816.6
Cumulative effect, period of adoption | Retained earnings
Summary of significant accounting policies
Cumulative effect of an accounting change		$ 0.0		$ (6.8)		$ 0.0	$ 0.0