Debt Securities (Tables)

3 Months Ended

Mar. 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
March 31, 2024
U.S. Treasury bonds	$	163.1	$	—	$	(5.9	)	$	157.2
Municipal bonds		1,381.2		7.1		(139.0	)		1,249.3
Foreign government bonds		209.9		0.2		(10.7	)		199.4
Governmental agency bonds		214.3		—		(13.9	)		200.4
Governmental agency mortgage-backed securities		4,316.7		2.2		(573.0	)		3,745.9
U.S. corporate debt securities		1,075.4		4.4		(58.6	)		1,021.2
Foreign corporate debt securities		498.4		4.0		(21.4	)		481.0
	$	7,859.0	$	17.9	$	(822.5	)	$	7,054.4
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
March 31, 2024
U.S. Treasury bonds	$	101.2	$	(1.3	)	$	50.4	$	(4.6	)	$	151.6	$	(5.9	)
Municipal bonds		133.1		(1.5	)		970.8		(137.5	)		1,103.9		(139.0	)
Foreign government bonds		68.3		(0.2	)		96.1		(10.5	)		164.4		(10.7	)
Governmental agency bonds		83.0		(0.6	)		117.4		(13.3	)		200.4		(13.9	)
Governmental agency mortgage-backed    securities		490.3		(15.4	)		3,103.7		(557.6	)		3,594.0		(573.0	)
U.S. corporate debt securities		127.1		(1.6	)		598.1		(57.0	)		725.2		(58.6	)
Foreign corporate debt securities		64.8		(0.6	)		257.7		(20.8	)		322.5		(21.4	)
	$	1,067.8	$	(21.2	)	$	5,194.2	$	(801.3	)	$	6,262.0	$	(822.5	)
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 March 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	$	5.7	$	136.9	$	0.4	$	20.1	$	163.1
Estimated fair value	$	5.7	$	132.1	$	0.3	$	19.1	$	157.2
Municipal bonds
Amortized cost		12.6		382.4		478.4		507.8		1,381.2
Estimated fair value		12.5		351.1		413.1		472.6		1,249.3
Foreign government bonds
Amortized cost		31.2		104.5		67.5		6.7		209.9
Estimated fair value		31.0		103.8		58.6		6.0		199.4
Governmental agency bonds
Amortized cost		—		166.6		1.6		46.1		214.3
Estimated fair value		—		161.6		1.5		37.3		200.4
U.S. corporate debt securities
Amortized cost		13.9		732.1		245.6		83.8		1,075.4
Estimated fair value		13.8		693.1		239.8		74.5		1,021.2
Foreign corporate debt securities
Amortized cost		31.9		313.5		121.2		31.8		498.4
Estimated fair value		31.3		302.6		118.5		28.6		481.0
Total debt securities (excluding mortgage-backed    securities)
Amortized cost	$	95.3	$	1,836.0	$	914.7	$	696.3	$	3,542.3
Estimated fair value	$	94.3	$	1,744.3	$	831.8	$	638.1	$	3,308.5
Total mortgage-backed securities
Amortized cost										4,316.7
Estimated fair value										3,745.9
Total debt securities
Amortized cost									$	7,859.0
Estimated fair value									$	7,054.4

Composition of Debt Securities Portfolio by Credit Rating Agencies

The composition of the debt securities portfolio at March 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	$	157.2		100.0	%	$	—		—	%	$	—		—	%	$	157.2
Municipal bonds		1,223.0		97.9			24.6		2.0			1.7		0.1			1,249.3
Foreign government bonds		193.9		97.2			4.7		2.4			0.8		0.4			199.4
Governmental agency bonds		200.4		100.0			—		—			—		—			200.4
Governmental agency mortgage-    backed securities		3,745.9		100.0			—		—			—		—			3,745.9
U.S. corporate debt securities		492.0		48.1			380.5		37.3			148.7		14.6			1,021.2
Foreign corporate debt securities		251.9		52.3			193.2		40.2			35.9		7.5			481.0
	$	6,264.3		88.8	%	$	603.0		8.5	%	$	187.1		2.7	%	$	7,054.4

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 March 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	$	151.6		100.0	%	$	—		—	%	$	—		—	%	$	151.6
Municipal bonds		1,084.3		98.3			19.3		1.7			0.3		—			1,103.9
Foreign government bonds		162.3		98.7			1.3		0.8			0.8		0.5			164.4
Governmental agency bonds		200.4		100.0			—		—			—		—			200.4
Governmental agency mortgage-    backed securities		3,594.0		100.0			—		—			—		—			3,594.0
U.S. corporate debt securities		394.5		54.4			273.8		37.8			56.9		7.8			725.2
Foreign corporate debt securities		188.3		58.4			116.2		36.0			18.0		5.6			322.5
	$	5,775.4		92.2	%	$	410.6		6.6	%	$	76.0		1.2	%	$	6,262.0