Consolidated Statements of Comprehensive Income - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Net income	$ 90.9	$ 55.0
Other comprehensive income (loss)
Foreign currency translation adjustments	2.3	2.8
Unrealized gain on investments, net of tax	0.2	0.3
Defined benefit pension plans:
Amortization of amounts included in net periodic benefit cost, net of tax	1.2	1.1
Other comprehensive income	3.9	4.0
Comprehensive income	94.8	59.0
Actuarial loss, net of tax
Defined benefit pension plans:
Change benefit obligations, translation adjustment	0.2	(0.2)
Amortization of amounts included in net periodic benefit cost, net of tax	1.1	0.9
Other comprehensive income	1.3	0.7
Prior service cost, net of tax
Defined benefit pension plans:
Amortization of amounts included in net periodic benefit cost, net of tax	0.1	0.2
Other comprehensive income	$ 0.1	$ 0.2