Schedule II – Condensed Financial Statements (Parent Only) - Comprehensive Income (Additional Information) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Condensed Financial Statements, Captions [Line Items]
Change in unrealized gains on securities, tax	$ (52.6)	$ 196.7	$ 233.7
Change in unrealized gains on derivative transactions, tax	0.3	0.7	0.7
Change in foreign currency translation, tax	(2.3)	(6.0)	3.1
Amortization of pension and postretirement unrecognized net periodic benefit cost and change in funded status, tax	7.2	(0.9)	(3.8)
Parent Company
Condensed Financial Statements, Captions [Line Items]
Change in unrealized gains on securities, tax	(3.4)	4.5	1.2
Change in unrealized gains on derivative transactions, tax	0.3	0.7	0.6
Change in foreign currency translation, tax	0.0	0.4	(0.4)
Amortization of pension and postretirement unrecognized net periodic benefit cost and change in funded status, tax	$ 7.2	$ (0.8)	$ (3.9)