CONDENSED CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME (unaudited) - USD ($) $ in Millions	3 Months Ended
	Jun. 30, 2018	Jun. 30, 2017
Statement of Comprehensive Income [Abstract]
Net income	$ 35.7	$ 170.6
Available-for-sale securities:
Unrealized holding losses, net of tax effect	(5.6)	(0.9)
Reclassification of realized transactions, net of tax effect	5.6	0.0
Defined benefit plans:
Actuarial gains (losses) related to defined benefit pension plans, net of tax (provision) benefit	4.4	(4.1)
Reclassification of realized transactions, net of tax effect	0.3	0.2
Change in net foreign currency translation adjustment	(0.3)	0.0
Other comprehensive income (loss), net of tax effect	4.4	(4.8)
Comprehensive income	$ 40.1	$ 165.8