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Investments (Tables)
 3 Months Ended
Mar. 31, 2024
Investments, Debt and Equity Securities [Abstract]  
Schedule of Investments Available for Sale
Investments available for sale consist of the following:
March 31, 2024 (In thousands)Amortized
Cost		Unrealized
Gains		Unrealized
Losses		Fair
Value
U.S. Treasury securities$1,057,616 $187 $(21,016)$1,036,787 
U.S. agency securities1,574 — (1)1,573 
U.S. agency mortgage-backed securities901,725 105 (114,545)787,285 
Municipal debt securities (1)581,274 3,595 (45,191)539,678 
Non-U.S. government securities77,443 — (11,048)66,395 
Corporate debt securities (2)1,311,746 2,079 (94,665)1,219,160 
Residential and commercial mortgage securities558,754 327 (52,301)506,780 
Asset-backed securities537,758 292 (15,698)522,352 
Money market funds689,064 — — 689,064 
Total investments available for sale$5,716,954 $6,585 $(354,465)$5,369,074 
December 31, 2023 (In thousands)Amortized
Cost		Unrealized
Gains		Unrealized
Losses		Fair
Value
U.S. Treasury securities$1,014,076 $1,434 $(19,128)$996,382 
U.S. agency securities7,199 — (4)7,195 
U.S. agency mortgage-backed securities922,907 438 (101,999)821,346 
Municipal debt securities (1)585,047 6,660 (44,449)547,258 
Non-U.S. government securities77,516 — (10,069)67,447 
Corporate debt securities (2)1,380,533 4,425 (87,903)1,297,055 
Residential and commercial mortgage securities571,163 286 (53,509)517,940 
Asset-backed securities584,168 203 (19,376)564,995 
Money market funds444,121 — — 444,121 
Total investments available for sale$5,586,730 $13,446 $(336,437)$5,263,739 

 March 31,December 31,
(1) The following table summarizes municipal debt securities as of :20242023
Special revenue bonds81.7 %81.4 %
General obligation bonds18.3 18.6 
Total100.0 %100.0 %

 March 31,December 31,
(2) The following table summarizes corporate debt securities as of :20242023
Financial41.8 %42.0 %
Consumer, non-cyclical15.9 15.9 
Industrial8.0 8.1 
Consumer, cyclical7.2 7.1 
Communications7.1 7.2 
Utilities6.4 6.3 
Technology6.2 6.2 
Energy4.9 4.7 
Basic Materials2.5 2.5 
Total100.0 %100.0 %
Schedule of Amortized Cost and Fair Value of Investments Available for Sale by Contractual Maturity
The amortized cost and fair value of investments available for sale at March 31, 2024, by contractual maturity, are shown below. Expected maturities will differ from contractual maturities because borrowers may have the right to call or prepay obligations with or without call or prepayment penalties. Because most U.S. agency mortgage-backed securities, residential and commercial mortgage securities and asset-backed securities provide for periodic payments throughout their lives, they are listed below in separate categories.
 
(In thousands)Amortized
Cost		Fair
Value
U.S. Treasury securities:  
Due in 1 year$633,987 $632,624 
Due after 1 but within 5 years370,211 355,791 
Due after 5 but within 10 years38,986 35,391 
Due after 10 years14,432 12,981 
Subtotal1,057,616 1,036,787 
U.S. agency securities:  
Due in 1 year1,574 1,573 
Subtotal1,574 1,573 
Municipal debt securities:  
Due in 1 year6,934 6,852 
Due after 1 but within 5 years87,058 84,352 
Due after 5 but within 10 years133,277 124,643 
Due after 10 years354,005 323,831 
Subtotal581,274 539,678 
Non-U.S. government securities:
Due in 1 year4,797 4,691 
Due after 1 but within 5 years31,909 30,432 
Due after 5 but within 10 years5,789 4,666 
Due after 10 years34,948 26,606 
Subtotal77,443 66,395 
Corporate debt securities:  
Due in 1 year169,890 168,104 
Due after 1 but within 5 years442,671 421,440 
Due after 5 but within 10 years552,880 509,315 
Due after 10 years146,305 120,301 
Subtotal1,311,746 1,219,160 
U.S. agency mortgage-backed securities901,725 787,285 
Residential and commercial mortgage securities558,754 506,780 
Asset-backed securities537,758 522,352 
Money market funds689,064 689,064 
Total investments available for sale$5,716,954 $5,369,074 
Schedule of Realized Gross Gains and Losses on Sale of Investments Available for Sale
The components of realized investment (losses) gains, net on the condensed consolidated statements of comprehensive income were as follows:
 
 Three Months Ended March 31,
(In thousands)20242023
Realized gross gains$3 $869 
Realized gross losses(1,143)(1,357)
Schedule of Fair Value of Investments in an Unrealized Loss Position and Related Unrealized Losses
The fair value of investments available for sale in an unrealized loss position and the related unrealized losses for which no allowance for credit loss has been recorded were as follows:
 
 Less than 12 months12 months or moreTotal
March 31, 2024 (In thousands)Fair
Value		Gross
Unrealized
Losses		Fair
Value		Gross
Unrealized
Losses		Fair
Value		Gross
Unrealized
Losses
U.S. Treasury securities$516,852 $(2,708)$318,693 $(18,308)$835,545 $(21,016)
U.S. agency securities1,573 (1)— — 1,573 (1)
U.S. agency mortgage-backed securities92,502 (1,363)683,424 (113,182)775,926 (114,545)
Municipal debt securities62,169 (882)318,055 (44,309)380,224 (45,191)
Non-U.S. government securities— — 66,395 (11,048)66,395 (11,048)
Corporate debt securities202,873 (2,606)888,500 (92,059)1,091,373 (94,665)
Residential and commercial mortgage securities
20,677 (323)473,205 (51,978)493,882 (52,301)
Asset-backed securities118,848 (511)286,205 (15,187)405,053 (15,698)
Total$1,015,494 $(8,394)$3,034,477 $(346,071)$4,049,971 $(354,465)
 
 Less than 12 months12 months or moreTotal
December 31, 2023 (In thousands)Fair
Value		Gross
Unrealized
Losses		Fair
Value		Gross
Unrealized
Losses		Fair
Value		Gross
Unrealized
Losses
U.S. Treasury securities$139,398 $(1,075)$355,921 $(18,053)$495,319 $(19,128)
U.S. agency securities5,572 (2)1,623 (2)7,195 (4)
U.S. agency mortgage-backed securities129,359 (1,616)654,018 (100,383)783,377 (101,999)
Municipal debt securities59,301 (987)297,039 (43,462)356,340 (44,449)
Non-U.S. government securities— — 67,447 (10,069)67,447 (10,069)
Corporate debt securities119,764 (733)905,606 (87,170)1,025,370 (87,903)
Residential and commercial mortgage securities
31,936 (999)459,789 (52,510)491,725 (53,509)
Asset-backed securities65,195 (347)459,324 (19,029)524,519 (19,376)
Total$550,525 $(5,759)$3,200,767 $(330,678)$3,751,292 $(336,437)
Schedule of Net Investment Income
Net investment income consists of: 
 Three Months Ended March 31,
(In thousands)20242023
Fixed maturities$46,288 $43,473 
Short-term investments7,143 1,721 
Gross investment income53,431 45,194 
Investment expenses(1,346)(1,958)
Net investment income$52,085 $43,236