Consolidated Statements of Comprehensive Income - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Statement of Comprehensive Income [Abstract]
Net Income	$ 598.6	$ 686.4	$ 1,534.3
Unrealized gains (losses) on securities not included in the statements of income:
Unrealized gains (losses) on securities before reclassifications, not included in the statements of income	285.3	(1,145.7)	(362.0)
Amounts reclassified as realized investment (gains) losses in the statements of income	184.5	312.3	(1.7)
Pretax unrealized gains (losses) on securities not included in the statements of income	469.8	(833.3)	(363.8)
Deferred income taxes (credits)	98.9	(175.9)	(76.8)
Net unrealized gains (losses) on securities not included in the statements of income, net of tax	370.8	(657.3)	(287.0)
Defined benefit pension plans:
Net pension adjustment before reclassifications	10.7	83.6	94.5
Amounts reclassified as underwriting, acquisition, and other expenses in the statements of income	0.1	3.0	7.4
Pretax net adjustment related to defined benefit pension plans	10.8	86.6	101.9
Deferred income taxes (credits)	2.2	18.2	21.4
Net adjustment related to defined benefit pension plans, net of tax	8.5	68.4	80.5
Foreign currency translation adjustment and other	6.0	(6.4)	1.3
Total other comprehensive income (loss)	385.4	(595.3)	(205.1)
Comprehensive Income	$ 984.1	$ 91.1	$ 1,329.2