Debt Securities (Tables)

9 Months Ended

Sep. 30, 2021

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:

(in thousands)	Amortized cost		Allowance for credit losses			Gross unrealized					Estimated fair value
						Gains		Losses
September 30, 2021
U.S. Treasury bonds	$	172,255	$	—		$	695	$	(1,207	)	$	171,743
Municipal bonds		1,564,102		—			60,705		(11,006	)		1,613,801
Foreign government bonds		232,495		—			2,209		(3,329	)		231,375
Governmental agency bonds		208,094		—			3,975		(562	)		211,507
Governmental agency mortgage-backed securities		5,360,175		—			44,912		(31,929	)		5,373,158
U.S. corporate debt securities		1,002,449		(101	)		23,760		(3,719	)		1,022,389
Foreign corporate debt securities		461,183		—			12,137		(1,487	)		471,833
	$	9,000,753	$	(101	)	$	148,393	$	(53,239	)	$	9,095,806
December 31, 2020
U.S. Treasury bonds	$	80,172	$	—		$	778	$	(104	)	$	80,846
Municipal bonds		1,168,425		—			80,953		(570	)		1,248,808
Foreign government bonds		194,042		—			6,004		(516	)		199,530
Governmental agency bonds		254,248		—			9,869		(195	)		263,922
Governmental agency mortgage-backed securities		3,401,737		—			74,549		(1,668	)		3,474,618
U.S. corporate debt securities		637,808		(119	)		43,505		(497	)		680,697
Foreign corporate debt securities		384,572		(13	)		22,078		(236	)		406,401
	$	6,121,004	$	(132	)	$	237,736	$	(3,786	)	$	6,354,822

Gross Unrealized Losses on Investments in Debt Securities

Investments in debt securities in an unrealized loss position, based on length of time, are as follows:

	Less than 12 months					12 months or longer					Total
(in thousands)	Estimated fair value		Unrealized losses			Estimated fair value		Unrealized losses			Estimated fair value		Unrealized losses
September 30, 2021
U.S. Treasury bonds	$	52,927	$	(916	)	$	6,518	$	(291	)	$	59,445	$	(1,207	)
Municipal bonds		570,038		(10,724	)		6,711		(282	)		576,749		(11,006	)
Foreign government bonds		125,918		(1,094	)		29,032		(2,235	)		154,950		(3,329	)
Governmental agency bonds		72,325		(469	)		3,222		(93	)		75,547		(562	)
Governmental agency mortgage-backed securities		3,070,056		(31,928	)		103		(1	)		3,070,159		(31,929	)
U.S. corporate debt securities		425,854		(3,388	)		6,848		(331	)		432,702		(3,719	)
Foreign corporate debt securities		161,416		(1,473	)		1,234		(14	)		162,650		(1,487	)
	$	4,478,534	$	(49,992	)	$	53,668	$	(3,247	)	$	4,532,202	$	(53,239	)
December 31, 2020
U.S. Treasury bonds	$	7,744	$	(104	)	$	—	$	—		$	7,744	$	(104	)
Municipal bonds		74,045		(570	)		—		—			74,045		(570	)
Foreign government bonds		67,094		(516	)		—		—			67,094		(516	)
Governmental agency bonds		15,353		(195	)		—		—			15,353		(195	)
Governmental agency mortgage-backed securities		287,947		(1,089	)		100,473		(579	)		388,420		(1,668	)
U.S. corporate debt securities		42,508		(484	)		1,357		(13	)		43,865		(497	)
Foreign corporate debt securities		19,042		(232	)		276		(4	)		19,318		(236	)
	$	513,733	$	(3,190	)	$	102,106	$	(596	)	$	615,839	$	(3,786	)

Rollforward of Allowance for Credit Losses for Investments in Debt Securities

Activity in the allowance for credit losses on debt securities for the three and nine months ended September 30, 2021 and 2020, is summarized as follows:

	Three Months Ended September 30,						Nine Months Ended September 30,
(in thousands)	2021			2020			2021			2020
Balance at beginning of period	$	(105	)	$	(1,380	)	$	(132	)	$	—
Credit losses recognized during the period		—			—			—			(7,493	)
Net (increases) decreases to credit losses previously recognized		(4	)		278			—			3,738
Reductions for securities sold/matured		8			271			31			2,924
Reclassification to assets previously held for sale		—			10			—			10
Balance at end of period	$	(101	)	$	(821	)	$	(101	)	$	(821	)

Investments in Debt Securities by Contractual Maturity

Investments in debt securities at September 30, 2021, by contractual maturities, are as follows:

(in thousands)	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	$	46,695	$	78,036	$	25,518	$	22,006	$	172,255
Estimated fair value	$	46,776	$	77,850	$	25,673	$	21,444	$	171,743
Municipal bonds
Amortized cost		26,737		97,493		751,040		688,832		1,564,102
Estimated fair value		26,884		102,020		771,806		713,091		1,613,801
Foreign government bonds
Amortized cost		52,799		93,254		76,214		10,228		232,495
Estimated fair value		52,826		94,105		74,215		10,229		231,375
Governmental agency bonds
Amortized cost		19,375		80,313		37,500		70,906		208,094
Estimated fair value		19,596		81,677		37,644		72,590		211,507
U.S. corporate debt securities
Amortized cost		36,886		479,958		414,830		70,775		1,002,449
Estimated fair value		37,292		492,943		417,154		75,000		1,022,389
Foreign corporate debt securities
Amortized cost		20,621		266,364		134,173		40,025		461,183
Estimated fair value		20,876		272,278		136,133		42,546		471,833
Total debt securities excluding mortgage-backed securities
Amortized cost	$	203,113	$	1,095,418	$	1,439,275	$	902,772	$	3,640,578
Estimated fair value	$	204,250	$	1,120,873	$	1,462,625	$	934,900	$	3,722,648
Total mortgage-backed securities
Amortized cost										5,360,175
Estimated fair value										5,373,158
Total debt securities
Amortized cost									$	9,000,753
Estimated fair value									$	9,095,806

Composition of Debt Securities Portfolio by Credit Rating Agencies

The composition of the debt securities portfolio at September 30, 2021, by credit rating, is as follows:

	A- or higher				BBB+ to BBB-				Non-Investment Grade				Total
(in thousands, except percentages)	Estimated fair value		Percentage		Estimated fair value		Percentage		Estimated fair value		Percentage		Estimated fair value
Debt securities:
U.S. Treasury bonds	$	171,743		100.0	$	—		—	$	—		—	$	171,743
Municipal bonds		1,577,214		97.7		36,587		2.3		—		—		1,613,801
Foreign government bonds		218,100		94.2		10,088		4.4		3,187		1.4		231,375
Governmental agency bonds		211,507		100.0		—		—		—		—		211,507
Governmental agency mortgage-backed securities		5,373,158		100.0		—		—		—		—		5,373,158
U.S. corporate debt securities		435,436		42.6		403,668		39.5		183,285		17.9		1,022,389
Foreign corporate debt securities		200,539		42.5		222,298		47.1		48,996		10.4		471,833
	$	8,187,697		90.0	$	672,641		7.4	$	235,468		2.6	$	9,095,806

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 September 30, 2021, by credit rating, is as follows:

	A- or higher				BBB+ to BBB-				Non-Investment Grade				Total
(in thousands, except percentages)	Estimated fair value		Percentage		Estimated fair value		Percentage		Estimated fair value		Percentage		Estimated fair value
U.S. Treasury bonds	$	59,445		100.0	$	—		—	$	—		—	$	59,445
Municipal bonds		563,613		97.7		13,136		2.3		—		—		576,749
Foreign government bonds		153,987		99.4		—		—		963		0.6		154,950
Governmental agency bonds		75,547		100.0		—		—		—		—		75,547
Governmental agency mortgage-backed securities		3,070,159		100.0		—		—		—		—		3,070,159
U.S. corporate debt securities		233,772		54.0		127,998		29.6		70,932		16.4		432,702
Foreign corporate debt securities		94,410		58.0		47,163		29.0		21,077		13.0		162,650
	$	4,250,933		93.7	$	188,297		4.2	$	92,972		2.1	$	4,532,202