Consolidated Statements of Comprehensive Income - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Statement of Comprehensive Income [Abstract]
Net Income	$ 686.4	$ 1,534.3	$ 558.6
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	(1,145.7)	(362.0)	335.5
Amounts reclassified as realized investment (gains) losses in the statements of income	312.3	(1.7)	7.1
Pretax unrealized gains (losses) on securities not included in the statements of income	(833.3)	(363.8)	342.7
Deferred income taxes (credits)	(175.9)	(76.8)	72.3
Net unrealized gains (losses) on securities not included in the statements of income, net of tax	(657.3)	(287.0)	270.3
Defined benefit pension plans:
Net pension adjustment before reclassifications	83.6	94.5	(88.4)
Amounts reclassified as underwriting, acquisition, and other expenses in the statements of income	3.0	7.4	3.6
Pretax net adjustment related to defined benefit pension plans	86.6	101.9	(84.8)
Deferred income taxes (credits)	18.2	21.4	(17.8)
Net adjustment related to defined benefit pension plans, net of tax	68.4	80.5	(67.0)
Foreign currency translation adjustment	(11.8)	0.4	2.9
Total other comprehensive income (loss)	(600.7)	(206.0)	206.3
Comprehensive Income	$ 85.7	$ 1,328.3	$ 765.0