Investments (Tables)

12 Months Ended

Dec. 31, 2023

Investments, Debt and Equity Securities [Abstract]

Amortized Cost and Estimated Fair Values Of Fixed Maturities

	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses		Estimated Fair Value
Fixed Income Securities by Type:
December 31, 2023:
Government & Agency	$	1,920.3	$	3.2	$	64.6	$	1,858.9
Municipal	774.5		0.2		7.1		767.6
Corporate	9,568.1		135.5		190.3		9,513.3
	$	12,263.0	$	139.0	$	262.0	$	12,139.9
December 31, 2022:
Government & Agency	$	2,300.0	$	—	$	114.8	$	2,185.2
Municipal	896.9		0.1		15.5		881.5
Corporate	9,139.3		20.3		479.6		8,680.0
	$	12,336.3	$	20.5	$	610.1	$	11,746.7

Amortized Cost and Estimated Fair Values Of Fixed Maturities By Contractual Maturities

	Amortized Cost		Estimated Fair Value
Fixed Income Securities Stratified by Contractual Maturity at December 31, 2023:
Due in one year or less	$	1,604.0	$	1,587.7
Due after one year through five years	6,119.3		5,999.4
Due after five years through ten years	4,455.6		4,467.7
Due after ten years	83.9		84.9
	$	12,263.0	$	12,139.9

Gross Unrealized Losses and Fair Value, Aggregated

	Less than 12 Months				12 Months or Greater				Total
	Fair Value		Unrealized Losses		Fair Value		Unrealized Losses		Fair Value		Unrealized Losses
December 31, 2023:
Fixed Income Securities:
Government & Agency	$	461.0	$	2.7	$	1,179.3	$	61.8	$	1,640.4	$	64.6
Municipal	173.1		0.8		554.7		6.2		727.9		7.1
Corporate	853.3		8.2		4,270.9		182.0		5,124.3		190.3
	$	1,487.6	$	11.8	$	6,005.1	$	250.2	$	7,492.7	$	262.0
December 31, 2022:
Fixed Income Securities:
Government & Agency	$	1,769.6	$	71.0	$	403.8	$	43.8	$	2,173.4	$	114.8
Municipal	845.6		13.0		9.8		2.5		855.5		15.5
Corporate	6,796.7		355.0		1,043.7		124.6		7,840.4		479.6
	$	9,412.0	$	439.1	$	1,457.4	$	170.9	$	10,869.5	$	610.1

Equity Securities Reflecting Reported Adjusted Cost

	Equity Securities
	Cost		Gross Unrealized Gains		Gross Unrealized Losses		Estimated Fair Value
December 31, 2023	$	1,511.9	$	1,164.7	$	15.7	$	2,660.8
December 31, 2022	$	1,948.1	$	1,291.5	$	18.6	$	3,220.9

Fair Value, Assets Measured on Recurring Basis

	Fair Value Measurements
As of December 31, 2023:	Level 1		Level 2		Level 3		Total
Fixed income securities:
Government & Agency	$	1,379.8	$	479.1	$	—	$	1,858.9
Municipal	—		767.6		—		767.6
Corporate	—		9,493.7		19.5		9,513.3
Short-term investments	1,032.6		—		—		1,032.6
Equity securities	$	2,653.8	$	—	$	7.0	$	2,660.8
As of December 31, 2022:
Fixed income securities:
Government & Agency	$	1,598.8	$	586.3	$	—	$	2,185.2
Municipal	—		881.5		—		881.5
Corporate	—		8,659.2		20.8		8,680.0
Short-term investments	860.8		—		—		860.8
Equity securities	$	3,219.1	$	—	$	1.7	$	3,220.9

Investment Income

Years Ended December 31:	2023		2022		2021
Investment income from:
Fixed income securities	$	438.8	$	314.4	$	280.6
Equity securities	92.1		132.5		157.5
Short-term investments	50.9		17.9		0.1
Other investments (a)	17.0		4.3		2.1
Gross investment income	598.9		469.3		440.4
Investment expenses (a)	20.6		9.7		6.1
Net investment income	$	578.3	$	459.5	$	434.3
Net investment gains (losses):
Realized from actual transactions:
Fixed income securities:
Gains	$	1.2	$	2.6	$	3.4
Losses	(181.9)		(190.2)		(1.9)
Net	(180.7)		(187.6)		1.5
Equity securities:
Gains	214.5		486.5		68.0
Losses	(51.4)		(111.9)		(62.8)
Net	163.0		374.5		5.1
Other investments, net	2.4		(1.2)		0.2
Total realized from actual transactions	(15.2)		185.7		6.9
From impairments (b)	(51.8)		(123.5)		—
From unrealized changes in fair value of equity securities	(123.9)		(263.4)		751.1
Total realized and unrealized investment gains (losses)	(190.9)		(201.1)		758.0
Current and deferred income taxes (credits)	(40.0)		(42.5)		159.6
Net of tax realized and unrealized investment gains (losses)	$	(150.8)	$	(158.6)	$	598.4
Changes in unrealized investment gains (losses)
reflected directly in shareholders' equity on:
Fixed income securities	$	464.1	$	(824.7)	$	(361.2)
Less: Deferred income taxes (credits)	97.7		(174.1)		(76.2)
	366.3		(650.5)		(284.9)
Other investments	5.7		(8.5)		(2.5)
Less: Deferred income taxes (credits)	1.1		(1.8)		(0.5)
	4.5		(6.7)		(2.0)
Net changes in unrealized investment gains (losses), net of tax	$	370.8	$	(657.3)	$	(287.0)

_________

(a)     Includes interest on funds held.

(b) Includes expected loss on the pending sale of RMICC. See Note 2 for further discussion.