Consolidated Statements of Comprehensive Income - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Statement Of Income And Comprehensive Income [Abstract]
Net income	$ 100.4	$ 11.1	$ 267.4	$ 134.7
Available-for-sale securities:
Net (depreciation) appreciation during the period	(25.0)	1.7	(197.9)	40.0
Change in other-than-temporary impairment losses recognized in other comprehensive (loss) income	(0.2)	2.0	0.1	2.5
Total available-for-sale securities	(25.2)	3.7	(197.8)	42.5
Pension and postretirement benefits:
Net change in net actuarial loss and prior service cost	1.9	2.3	0.5	7.0
Cumulative foreign currency translation adjustment:
Amount recognized as cumulative foreign currency translation during the period	1.6	1.1	0.6	1.7
Total other comprehensive (loss) income, net of tax	(21.7)	7.1	(196.7)	51.2
Comprehensive income	$ 78.7	$ 18.2	$ 70.7	$ 185.9