Consolidated Statements of Comprehensive Income - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Statement Of Income And Comprehensive Income [Abstract]
Net income	$ 425.1	$ 391.0	$ 186.2
Available-for-sale securities:
Net appreciation (depreciation) during the period	238.8	(170.1)	13.4
Change in other-than-temporary impairment losses recognized in other comprehensive income	2.8		6.0
Amount realized with sale of Chaucer business	0.1	19.1
Total available-for-sale securities	241.7	(151.0)	19.4
Pension and postretirement benefits:
Net actuarial gains (losses) and prior service costs arising in the period	16.2	(17.0)	13.6
Amortization recognized as net periodic benefit and postretirement cost	9.0	7.7	9.4
Amount realized with sale of Chaucer business		16.4
Total pension and postretirement benefits	25.2	7.1	23.0
Cumulative foreign currency translation adjustment:
Amount recognized as cumulative foreign currency translation during the period		(1.7)	2.4
Amount realized with sale of Chaucer business	0.7	23.2
Total cumulative foreign currency translation adjustment	0.7	21.5	2.4
Total other comprehensive income (loss), net of tax	267.6	(122.4)	44.8
Comprehensive income	$ 692.7	$ 268.6	$ 231.0