Debt Securities (Tables)

12 Months Ended

Dec. 31, 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
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
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
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	)
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 December 31, 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	$	32.9	$	117.6	$	4.7	$	25.5	$	180.7
Estimated fair value	$	32.8	$	114.7	$	4.6	$	23.5	$	175.6
Municipal bonds
Amortized cost		7.1		83.0		283.1		471.7		844.9
Estimated fair value		7.1		80.5		247.6		441.3		776.5
Foreign government bonds
Amortized cost		13.0		123.7		73.7		6.7		217.1
Estimated fair value		13.0		125.1		67.9		5.6		211.6
Governmental agency bonds
Amortized cost		8.6		1.4		149.5		44.3		203.8
Estimated fair value		8.6		1.4		144.2		35.6		189.8
U.S. corporate debt securities
Amortized cost		12.4		545.2		295.6		95.2		948.4
Estimated fair value		12.5		539.0		290.5		83.6		925.6
Foreign corporate debt securities
Amortized cost		22.1		309.9		128.4		31.2		491.6
Estimated fair value		22.0		308.4		126.1		28.0		484.5
Total debt securities, excluding mortgage-backed    securities
Amortized cost	$	96.1	$	1,180.8	$	935.0	$	674.6	$	2,886.5
Estimated fair value	$	96.0	$	1,169.1	$	880.9	$	617.6	$	2,763.6
Total mortgage-backed securities
Amortized cost										4,844.4
Estimated fair value										4,502.3
Total debt securities
Amortized cost									$	7,730.9
Estimated fair value									$	7,265.9

Composition of Debt Securities Portfolio by Credit Rating Agencies

The composition of the debt securities portfolio at December 31, 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	$	175.6		100.0	%	$	—		—	%	$	—		—	%	$	175.6
Municipal bonds		755.1		97.3			21.1		2.7			0.3		—			776.5
Foreign government bonds		206.5		97.6			4.4		2.1			0.7		0.3			211.6
Governmental agency bonds		189.8		100.0			—		—			—		—			189.8
Governmental agency    mortgage-backed securities		4,502.3		100.0			—		—			—		—			4,502.3
U.S. corporate debt securities		444.1		48.0			314.5		34.0			167.0		18.0			925.6
Foreign corporate debt securities		259.2		53.5			181.0		37.4			44.3		9.1			484.5
	$	6,532.6		89.9	%	$	521.0		7.2	%	$	212.3		2.9	%	$	7,265.9

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 December 31, 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	$	134.7		100.0	%	$	—		—	%	$	—		—	%	$	134.7
Municipal bonds		649.2		97.6			15.8		2.4			0.3		—			665.3
Foreign government bonds		97.4		95.0			4.4		4.3			0.7		0.7			102.5
Governmental agency bonds		182.3		100.0			—		—			—		—			182.3
Governmental agency    mortgage-backed securities		4,267.6		100.0			—		—			—		—			4,267.6
U.S. corporate debt securities		317.2		57.4			202.4		36.6			33.1		6.0			552.7
Foreign corporate debt securities		165.1		61.2			89.9		33.3			14.9		5.5			269.9
	$	5,813.5		94.1	%	$	312.5		5.1	%	$	49.0		0.8	%	$	6,175.0