Debt Securities (Tables)

6 Months Ended

Jun. 30, 2024

Investments, Debt and Equity Securities [Abstract]

Investments in Debt Securities, Classified as Available-For-Sale

Investments in debt securities, classified as available-for-sale, are as follows:

	Amortized		Gross unrealized					Estimated
(in millions)	cost		Gains		Losses			fair value
June 30, 2024
U.S. Treasury bonds	$	163.5	$	0.1	$	(6.1	)	$	157.5
Municipal bonds		1,382.3		5.9		(143.2	)		1,245.0
Foreign government bonds		223.4		0.4		(10.1	)		213.7
Governmental agency bonds		213.7		—		(13.9	)		199.8
Governmental agency mortgage-backed securities		4,315.9		1.6		(575.4	)		3,742.1
U.S. corporate debt securities		1,095.7		4.0		(58.1	)		1,041.6
Foreign corporate debt securities		498.2		3.6		(20.8	)		481.0
	$	7,892.7	$	15.6	$	(827.6	)	$	7,080.7
December 31, 2023
U.S. Treasury bonds	$	203.3	$	0.5	$	(4.5	)	$	199.3
Municipal bonds		1,373.7		8.8		(136.7	)		1,245.8
Foreign government bonds		228.4		1.4		(10.5	)		219.3
Governmental agency bonds		207.7		0.2		(12.5	)		195.4
Governmental agency mortgage-backed securities		4,396.2		6.3		(526.8	)		3,875.7
U.S. corporate debt securities		1,007.0		6.6		(55.2	)		958.4
Foreign corporate debt securities		478.9		5.8		(21.1	)		463.6
	$	7,895.2	$	29.6	$	(767.3	)	$	7,157.5

Gross Unrealized Losses on Investments in Debt Securities

Investments in debt securities in an unrealized loss position, and their respective length of time in such position, are as follows:

	Less than 12 months					12 months or longer					Total
(in millions)	Estimated fair value		Unrealized losses			Estimated fair value		Unrealized losses			Estimated fair value		Unrealized losses
June 30, 2024
U.S. Treasury bonds	$	94.9	$	(1.6	)	$	55.7	$	(4.5	)	$	150.6	$	(6.1	)
Municipal bonds		99.3		(1.4	)		1,010.8		(141.8	)		1,110.1		(143.2	)
Foreign government bonds		57.3		(0.1	)		81.8		(10.0	)		139.1		(10.1	)
Governmental agency bonds		82.5		(0.7	)		117.3		(13.2	)		199.8		(13.9	)
Governmental agency mortgage-backed    securities		363.7		(3.7	)		3,283.7		(571.7	)		3,647.4		(575.4	)
U.S. corporate debt securities		161.3		(1.9	)		587.9		(56.2	)		749.2		(58.1	)
Foreign corporate debt securities		68.5		(0.8	)		250.2		(20.0	)		318.7		(20.8	)
	$	927.5	$	(10.2	)	$	5,387.4	$	(817.4	)	$	6,314.9	$	(827.6	)
December 31, 2023
U.S. Treasury bonds	$	8.2	$	(0.1	)	$	55.4	$	(4.4	)	$	63.6	$	(4.5	)
Municipal bonds		107.4		(0.9	)		956.8		(135.8	)		1,064.2		(136.7	)
Foreign government bonds		33.3		(0.1	)		101.4		(10.4	)		134.7		(10.5	)
Governmental agency bonds		0.4		—			118.9		(12.5	)		119.3		(12.5	)
Governmental agency mortgage-backed    securities		338.3		(6.6	)		3,225.3		(520.2	)		3,563.6		(526.8	)
U.S. corporate debt securities		45.1		(0.4	)		602.5		(54.8	)		647.6		(55.2	)
Foreign corporate debt securities		19.3		(0.1	)		267.3		(21.0	)		286.6		(21.1	)
	$	552.0	$	(8.2	)	$	5,327.6	$	(759.1	)	$	5,879.6	$	(767.3	)

Investments in Debt Securities by Contractual Maturity

Investments in debt securities at June 30, 2024, by contractual maturities, are as follows:

(in millions)	Due in one year or less		Due after one through five years		Due after five through ten years		Due after ten years		Total
U.S. Treasury bonds
Amortized cost	$	6.7	$	137.0	$	4.7	$	15.1	$	163.5
Estimated fair value	$	6.7	$	132.3	$	4.7	$	13.8	$	157.5
Municipal bonds
Amortized cost		13.1		403.4		462.0		503.8		1,382.3
Estimated fair value		12.9		370.2		393.3		468.6		1,245.0
Foreign government bonds
Amortized cost		8.9		139.2		68.7		6.6		223.4
Estimated fair value		8.8		138.9		60.2		5.8		213.7
Governmental agency bonds
Amortized cost		36.3		130.3		1.5		45.6		213.7
Estimated fair value		35.6		126.1		1.4		36.7		199.8
U.S. corporate debt securities
Amortized cost		18.7		723.7		272.6		80.7		1,095.7
Estimated fair value		18.6		686.5		266.5		70.0		1,041.6
Foreign corporate debt securities
Amortized cost		28.7		312.5		125.0		32.0		498.2
Estimated fair value		28.2		302.5		121.9		28.4		481.0
Total debt securities (excluding mortgage-backed    securities)
Amortized cost	$	112.4	$	1,846.1	$	934.5	$	683.8	$	3,576.8
Estimated fair value	$	110.8	$	1,756.5	$	848.0	$	623.3	$	3,338.6
Total mortgage-backed securities
Amortized cost										4,315.9
Estimated fair value										3,742.1
Total debt securities
Amortized cost									$	7,892.7
Estimated fair value									$	7,080.7

Composition of Debt Securities Portfolio by Credit Rating Agencies

The composition of the debt securities portfolio at June 30, 2024, by credit rating, is as follows:

	A- or higher					BBB+ to BBB-					Non-Investment Grade					Total
(dollars in millions)	Estimated fair value		Percentage			Estimated fair value		Percentage			Estimated fair value		Percentage			Estimated fair value
U.S. Treasury bonds	$	157.5		100.0	%	$	—		—	%	$	—		—	%	$	157.5
Municipal bonds		1,220.9		98.1			22.4		1.8			1.7		0.1			1,245.0
Foreign government bonds		208.4		97.5			4.6		2.2			0.7		0.3			213.7
Governmental agency bonds		199.8		100.0			—		—			—		—			199.8
Governmental agency mortgage-    backed securities		3,742.1		100.0			—		—			—		—			3,742.1
U.S. corporate debt securities		494.8		47.5			386.0		37.1			160.8		15.4			1,041.6
Foreign corporate debt securities		250.9		52.1			190.8		39.7			39.3		8.2			481.0
	$	6,274.4		88.6	%	$	603.8		8.5	%	$	202.5		2.9	%	$	7,080.7

Composition of Debt Securities Portfolio in Unrealized Loss Position by Credit Rating Agencies

The composition of the debt securities portfolio in an unrealized loss position at June 30, 2024, by credit rating, is as follows:

	A- or higher					BBB+ to BBB-					Non-Investment Grade					Total
(dollars in millions)	Estimated fair value		Percentage			Estimated fair value		Percentage			Estimated fair value		Percentage			Estimated fair value
U.S. Treasury bonds	$	150.6		100.0	%	$	—		—	%	$	—		—	%	$	150.6
Municipal bonds		1,092.5		98.4			17.3		1.6			0.3		—			1,110.1
Foreign government bonds		137.2		98.6			1.2		0.9			0.7		0.5			139.1
Governmental agency bonds		199.8		100.0			—		—			—		—			199.8
Governmental agency mortgage-    backed securities		3,647.4		100.0			—		—			—		—			3,647.4
U.S. corporate debt securities		400.7		53.5			295.5		39.4			53.0		7.1			749.2
Foreign corporate debt securities		184.7		58.0			117.7		36.9			16.3		5.1			318.7
	$	5,812.9		92.1	%	$	431.7		6.8	%	$	70.3		1.1	%	$	6,314.9