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Unrecognized tax benefits (Details) - USD ($)
$ in Millions
12 Months Ended
Mar. 31, 2019
Mar. 31, 2018
Mar. 31, 2017
Mar. 31, 2016
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]        
Beginning balance $ 436.0 $ 398.5 $ 220.7 $ 170.7
Increases related to acquisitions   0.0 193.3 46.2
Decreases related to settlements with tax authorities   (0.1) (11.7) (8.0)
Decreases related to statute of limitation expirations   (10.9) (7.6) (4.6)
Increases related to current year tax positions   30.3 26.3 16.4
Increases (decreases) related to prior year tax positions     (22.5)  
Increases related to prior year tax positions   18.2   0.0
Ending balance   436.0 398.5 220.7
Unrecognized Tax Benefits, Income Tax Penalties and Interest Accrued [Abstract]        
Unrecognized tax benefits, interest on income taxes accrued   12.9 9.4  
Unrecognized tax benefits, income tax penalties accrued   67.9 66.1  
Unrecognized tax benefits, income tax penalties and interest expense   $ 5.4 $ 5.8 $ 1.7
Scenario, Forecast        
Income Tax Contingency [Line Items]        
Unrecognized tax benefits, potential decrease $ 20.0