Consolidated Statements of Comprehensive Income - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Statement Of Income And Comprehensive Income [Abstract]
Net income	$ 358.7	$ 425.1	$ 391.0
Changes in net unrealized gains (losses) on investment securities:
Having no credit losses recognized in the Consolidated Statements of Income	210.6	238.8	(170.1)
Having credit losses recognized in the Consolidated Statements of Income	1.5	2.8
Amount realized with sale of Chaucer business		0.1	19.1
Total available-for-sale securities	212.1	241.7	(151.0)
Pension and postretirement benefits:
Net actuarial gains (losses) and prior service costs arising in the period	3.1	16.2	(17.0)
Amortization recognized as net periodic benefit and postretirement cost	4.7	9.0	7.7
Amount realized with sale of Chaucer business			16.4
Total pension and postretirement benefits	7.8	25.2	7.1
Cumulative foreign currency translation adjustment:
Amount recognized as cumulative foreign currency translation during the period			(1.7)
Amount realized with sale of Chaucer business		0.7	23.2
Total cumulative foreign currency translation adjustment		0.7	21.5
Total other comprehensive income (loss), net of tax	219.9	267.6	(122.4)
Comprehensive income	$ 578.6	$ 692.7	$ 268.6