Debt Securities (Tables)

6 Months Ended

Jun. 30, 2025

Investments, Debt and Equity Securities [Abstract]

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

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

	Amortized		Gross unrealized					Estimated
(in millions)	cost		Gains		Losses			fair value
June 30, 2025
U.S. Treasury bonds	$	171.2	$	1.1	$	(2.1	)	$	170.2
Municipal bonds		946.6		4.6		(74.5	)		876.7
Foreign government bonds		229.8		1.5		(5.8	)		225.5
Governmental agency bonds		214.1		0.2		(8.0	)		206.3
Governmental agency mortgage-backed securities		5,265.4		17.9		(245.4	)		5,037.9
U.S. corporate debt securities		1,016.2		12.3		(18.5	)		1,010.0
Foreign corporate debt securities		490.9		8.6		(7.2	)		492.3
	$	8,334.2	$	46.2	$	(361.5	)	$	8,018.9
December 31, 2024
U.S. Treasury bonds	$	180.7	$	0.1	$	(5.2	)	$	175.6
Municipal bonds		844.9		5.3		(73.7	)		776.5
Foreign government bonds		217.1		1.6		(7.1	)		211.6
Governmental agency bonds		203.8		—		(14.0	)		189.8
Governmental agency mortgage-backed securities		4,844.4		1.7		(343.8	)		4,502.3
U.S. corporate debt securities		948.4		5.6		(28.4	)		925.6
Foreign corporate debt securities		491.6		5.3		(12.4	)		484.5
	$	7,730.9	$	19.6	$	(484.6	)	$	7,265.9

Sales of debt securities resulted in realized gains of $1.4 million and $2.5 million, realized losses of $6.8 million and $9.4 million and proceeds of $240.2 million and $507.0 million for the three and six months ended June 30, 2025, respectively. Sales of debt securities resulted in realized gains of $0.3 million and $1.1 million, realized losses of $1.3 million and $5.5 million and proceeds of $137.4 million and $361.9 million for the three and six months ended June 30, 2024, respectively.

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, 2025
U.S. Treasury bonds	$	9.4	$	(0.2	)	$	41.8	$	(1.9	)	$	51.2	$	(2.1	)
Municipal bonds		228.1		(9.5	)		391.6		(65.0	)		619.7		(74.5	)
Foreign government bonds		29.7		—			62.1		(5.8	)		91.8		(5.8	)
Governmental agency bonds		104.1		(0.4	)		37.2		(7.6	)		141.3		(8.0	)
Governmental agency mortgage-backed    securities		2,290.7		(47.0	)		1,246.4		(198.4	)		3,537.1		(245.4	)
U.S. corporate debt securities		258.6		(2.9	)		105.6		(15.6	)		364.2		(18.5	)
Foreign corporate debt securities		96.7		(0.6	)		87.9		(6.6	)		184.6		(7.2	)
	$	3,017.3	$	(60.6	)	$	1,972.6	$	(300.9	)	$	4,989.9	$	(361.5	)
December 31, 2024
U.S. Treasury bonds	$	81.8	$	(1.8	)	$	52.9	$	(3.4	)	$	134.7	$	(5.2	)
Municipal bonds		248.3		(4.4	)		417.0		(69.3	)		665.3		(73.7	)
Foreign government bonds		29.8		(0.2	)		72.7		(6.9	)		102.5		(7.1	)
Governmental agency bonds		144.4		(5.3	)		37.9		(8.7	)		182.3		(14.0	)
Governmental agency mortgage-backed    securities		2,977.2		(98.4	)		1,290.4		(245.4	)		4,267.6		(343.8	)
U.S. corporate debt securities		435.2		(9.6	)		117.5		(18.8	)		552.7		(28.4	)
Foreign corporate debt securities		159.4		(3.2	)		110.5		(9.2	)		269.9		(12.4	)
	$	4,076.1	$	(122.9	)	$	2,098.9	$	(361.7	)	$	6,175.0	$	(484.6	)

Investments in Debt Securities by Contractual Maturity

Investments in debt securities at June 30, 2025, 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	$	33.1	$	107.8	$	5.4	$	24.9	$	171.2
Estimated fair value	$	32.9	$	107.4	$	5.3	$	24.6	$	170.2
Municipal bonds
Amortized cost		7.2		94.3		359.3		485.8		946.6
Estimated fair value		7.2		91.9		331.0		446.6		876.7
Foreign government bonds
Amortized cost		16.4		164.4		45.3		3.7		229.8
Estimated fair value		16.4		163.1		43.3		2.7		225.5
Governmental agency bonds
Amortized cost		1.2		1.4		168.8		42.7		214.1
Estimated fair value		1.2		1.4		168.6		35.1		206.3
U.S. corporate debt securities
Amortized cost		12.0		558.2		354.5		91.5		1,016.2
Estimated fair value		11.9		560.8		356.0		81.3		1,010.0
Foreign corporate debt securities
Amortized cost		19.4		318.4		123.3		29.8		490.9
Estimated fair value		19.4		321.4		124.3		27.2		492.3
Total debt securities (excluding mortgage-backed    securities)
Amortized cost	$	89.3	$	1,244.5	$	1,056.6	$	678.4	$	3,068.8
Estimated fair value	$	89.0	$	1,246.0	$	1,028.5	$	617.5	$	2,981.0
Total mortgage-backed securities
Amortized cost										5,265.4
Estimated fair value										5,037.9
Total debt securities
Amortized cost									$	8,334.2
Estimated fair value									$	8,018.9

Composition of Debt Securities Portfolio by Credit Rating Agencies

The composition of the debt securities portfolio at June 30, 2025, 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	$	170.2		100.0	%	$	—		—	%	$	—		—	%	$	170.2
Municipal bonds		856.7		97.8			19.7		2.2			0.3		—			876.7
Foreign government bonds		220.3		97.7			4.5		2.0			0.7		0.3			225.5
Governmental agency bonds		206.3		100.0			—		—			—		—			206.3
Governmental agency mortgage-    backed securities		5,037.9		100.0			—		—			—		—			5,037.9
U.S. corporate debt securities		526.1		52.0			312.6		31.0			171.3		17.0			1,010.0
Foreign corporate debt securities		253.5		51.5			196.6		39.9			42.2		8.6			492.3
	$	7,271.0		90.6	%	$	533.4		6.7	%	$	214.5		2.7	%	$	8,018.9

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, 2025, 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	$	51.2		100.0	%	$	—		—	%	$	—		—	%	$	51.2
Municipal bonds		604.8		97.6			14.6		2.4			0.3		—			619.7
Foreign government bonds		89.6		97.6			1.5		1.6			0.7		0.8			91.8
Governmental agency bonds		141.3		100.0			—		—			—		—			141.3
Governmental agency mortgage-    backed securities		3,537.1		100.0			—		—			—		—			3,537.1
U.S. corporate debt securities		196.4		53.9			131.0		36.0			36.8		10.1			364.2
Foreign corporate debt securities		114.8		62.1			56.6		30.7			13.2		7.2			184.6
	$	4,735.2		94.9	%	$	203.7		4.1	%	$	51.0		1.0	%	$	4,989.9