Debt and Equity Securities (Tables)

3 Months Ended

Mar. 31, 2018

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

(in thousands)	Amortized cost		Gross unrealized					Estimated fair value
(in thousands)	Amortized cost		Gains		Losses			Estimated fair value
March 31, 2018
U.S. Treasury bonds	$	189,322	$	602	$	(3,146	)	$	186,778
Municipal bonds		1,104,512		6,013		(18,648	)		1,091,877
Foreign government bonds		155,001		176		(2,151	)		153,026
Governmental agency bonds		223,473		680		(4,471	)		219,682
Governmental agency mortgage-backed securities		2,325,348		2,692		(33,744	)		2,294,296
U.S. corporate debt securities		731,649		4,721		(10,054	)		726,316
Foreign corporate debt securities		263,396		1,631		(2,892	)		262,135
	$	4,992,701	$	16,515	$	(75,106	)	$	4,934,110
December 31, 2017
U.S. Treasury bonds	$	173,049	$	2,199	$	(1,250	)	$	173,998
Municipal bonds		1,031,146		12,185		(7,394	)		1,035,937
Foreign government bonds		170,220		489		(1,221	)		169,488
Governmental agency bonds		212,731		1,061		(2,322	)		211,470
Governmental agency mortgage-backed securities		2,172,377		3,168		(16,588	)		2,158,957
U.S. corporate debt securities		734,409		11,768		(2,962	)		743,215
Foreign corporate debt securities		256,430		4,145		(956	)		259,619
	$	4,750,362	$	35,015	$	(32,693	)	$	4,752,684

Gross unrealized losses on investments in debt securities 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
March 31, 2018
U.S. Treasury bonds	$	109,820	$	(2,009	)	$	38,554	$	(1,137	)	$	148,374	$	(3,146	)
Municipal bonds		503,940		(6,441	)		217,817		(12,207	)		721,757		(18,648	)
Foreign government bonds		93,606		(1,397	)		13,963		(754	)		107,569		(2,151	)
Governmental agency bonds		97,136		(1,542	)		93,289		(2,929	)		190,425		(4,471	)
Governmental agency mortgage-backed securities		976,932		(15,420	)		766,336		(18,324	)		1,743,268		(33,744	)
U.S. corporate debt securities		383,413		(7,256	)		56,030		(2,798	)		439,443		(10,054	)
Foreign corporate debt securities		154,534		(2,604	)		5,959		(288	)		160,493		(2,892	)
	$	2,319,381	$	(36,669	)	$	1,191,948	$	(38,437	)	$	3,511,329	$	(75,106	)
December 31, 2017
U.S. Treasury bonds	$	78,605	$	(511	)	$	37,498	$	(739	)	$	116,103	$	(1,250	)
Municipal bonds		279,292		(1,714	)		226,895		(5,680	)		506,187		(7,394	)
Foreign government bonds		98,942		(972	)		6,678		(249	)		105,620		(1,221	)
Governmental agency bonds		55,707		(409	)		93,737		(1,913	)		149,444		(2,322	)
Governmental agency mortgage-backed securities		671,871		(4,868	)		774,959		(11,720	)		1,446,830		(16,588	)
U.S. corporate debt securities		171,817		(1,568	)		60,724		(1,394	)		232,541		(2,962	)
Foreign corporate debt securities		81,525		(821	)		5,697		(135	)		87,222		(956	)
	$	1,437,759	$	(10,863	)	$	1,206,188	$	(21,830	)	$	2,643,947	$	(32,693	)

Investments in debt securities at March 31, 2018, 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	$	37,530	$	55,856	$	44,730	$	51,206	$	189,322
Estimated fair value	$	37,388	$	54,931	$	43,790	$	50,669	$	186,778
Municipal bonds
Amortized cost	$	70,776	$	293,994	$	272,117	$	467,625	$	1,104,512
Estimated fair value	$	70,767	$	293,665	$	269,827	$	457,618	$	1,091,877
Foreign government bonds
Amortized cost	$	13,139	$	113,211	$	12,517	$	16,134	$	155,001
Estimated fair value	$	13,143	$	112,212	$	12,430	$	15,241	$	153,026
Governmental agency bonds
Amortized cost	$	31,347	$	78,487	$	65,136	$	48,503	$	223,473
Estimated fair value	$	31,210	$	76,941	$	64,331	$	47,200	$	219,682
U.S. corporate debt securities
Amortized cost	$	21,390	$	321,444	$	315,735	$	73,080	$	731,649
Estimated fair value	$	21,431	$	318,706	$	312,509	$	73,670	$	726,316
Foreign corporate debt securities
Amortized cost	$	18,674	$	137,519	$	94,093	$	13,110	$	263,396
Estimated fair value	$	18,701	$	136,902	$	93,212	$	13,320	$	262,135
Total debt securities excluding mortgage-backed securities
Amortized cost	$	192,856	$	1,000,511	$	804,328	$	669,658	$	2,667,353
Estimated fair value	$	192,640	$	993,357	$	796,099	$	657,718	$	2,639,814
Total mortgage-backed securities
Amortized cost									$	2,325,348
Estimated fair value									$	2,294,296
Total debt securities
Amortized cost									$	4,992,701
Estimated fair value									$	4,934,110

Investments in equity securities are as follows:

The composition of the investment portfolio at March 31, 2018, 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		Percentage
March 31, 2018
Debt securities:
U.S. Treasury bonds	$	186,778	100.0	$	—	—	$	—	—	$	186,778	100.0
Municipal bonds		1,012,141	92.7		59,627	5.5		20,109	1.8		1,091,877	100.0
Foreign government bonds		123,117	80.5		24,484	16.0		5,425	3.5		153,026	100.0
Governmental agency bonds		219,682	100.0		—	—		—	—		219,682	100.0
Governmental agency mortgage-backed securities		2,294,296	100.0		—	—		—	—		2,294,296	100.0
U.S. corporate debt securities		259,578	35.7		239,276	32.9		227,462	31.4		726,316	100.0
Foreign corporate debt securities		122,163	46.6		108,273	41.3		31,699	12.1		262,135	100.0
Total debt securities		4,217,755	85.5		431,660	8.7		284,695	5.8		4,934,110	100.0
Preferred stocks		—	—		13,369	73.3		4,869	26.7		18,238	100.0
Total	$	4,217,755	85.2	$	445,029	9.0	$	289,564	5.8	$	4,952,348	100.0

The composition of the debt securities portfolio in an unrealized loss position at March 31, 2018, 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		Percentage
March 31, 2018
U.S. Treasury bonds	$	148,374	100.0	$	—	—	$	—	—	$	148,374	100.0
Municipal bonds		683,529	94.7		36,359	5.0		1,869	0.3		721,757	100.0
Foreign government bonds		80,575	74.9		23,833	22.2		3,161	2.9		107,569	100.0
Governmental agency bonds		190,425	100.0		—	—		—	—		190,425	100.0
Governmental agency mortgage-backed securities		1,743,268	100.0		—	—		—	—		1,743,268	100.0
U.S. corporate debt securities		195,799	44.6		172,175	39.2		71,469	16.2		439,443	100.0
Foreign corporate debt securities		74,495	46.4		69,761	43.5		16,237	10.1		160,493	100.0
Total	$	3,116,465	88.8	$	302,128	8.6	$	92,736	2.6	$	3,511,329	100.0