Accumulated Other Comprehensive Loss - Schedule of Reclassifications of Recognized Transactions out of AOCI (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2020	Mar. 31, 2019	Mar. 31, 2018
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Other income, net									$ 3.2	$ (2.2)	$ (5.8)
Net income	$ 99.9	$ 311.1	$ 108.9	$ 50.7	$ 174.7	$ 49.2	$ 96.3	$ 35.7	570.6	355.9	255.4
Reclassification out of Accumulated Other Comprehensive Income
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net income									(0.8)	(6.6)	(16.0)
Reclassification out of Accumulated Other Comprehensive Income | Unrealized losses on available-for-sale securities
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Other income, net									0.0	(5.6)	(15.2)
Reclassification out of Accumulated Other Comprehensive Income | Amortization of actuarial loss
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Other income, net									$ (0.8)	$ (1.0)	$ (0.8)