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Debt Securities - Debt securities in an unrealized loss position (Details) - USD ($)
$ in Thousands
Sep. 30, 2023
Dec. 31, 2022
Fair Value    
Less than 12 months $ 49,649 $ 1,070,674
12 months or more 2,122,190 1,368,859
Total 2,171,839 2,439,533
Unrealized Losses    
Less than 12 months (1,237) (81,841)
Unrealized Losses (333,315) (229,363)
Total (334,552) (311,204)
Fair Value    
Less than 12 months   58,065
12 months or more 701,676 727,230
Total 701,676 785,295
Unrealized Losses    
Less than 12 months   (8,009)
12 months or more (180,938) (125,008)
Total (180,938) (133,017)
U.S. Treasury securities    
Fair Value    
Less than 12 months 0 74
12 months or more 50,130 113,987
Total 50,130 114,061
Unrealized Losses    
Less than 12 months 0 0
Unrealized Losses (1,392) (3,744)
Total (1,392) (3,744)
Obligations of U.S. government corporations and agencies    
Fair Value    
Less than 12 months 1,983 19,603
12 months or more 7,699 0
Total 9,682 19,603
Unrealized Losses    
Less than 12 months (6) (321)
Unrealized Losses (248) 0
Total (254) (321)
Obligations of states and political subdivisions    
Fair Value    
Less than 12 months 45,778 166,147
12 months or more 177,195 75,217
Total 222,973 241,364
Unrealized Losses    
Less than 12 months (1,219) (10,059)
Unrealized Losses (26,539) (16,016)
Total (27,758) (26,075)
Asset-backed securities    
Fair Value    
Less than 12 months 0 390,164
12 months or more 479,448 79,711
Total 479,448 469,875
Unrealized Losses    
Less than 12 months 0 (15,648)
Unrealized Losses (7,069) (4,035)
Total (7,069) (19,683)
Commercial mortgage-backed securities    
Fair Value    
Less than 12 months 0 89,428
12 months or more 100,300 18,966
Total 100,300 108,394
Unrealized Losses    
Less than 12 months 0 (12,623)
Unrealized Losses (19,817) (3,406)
Total (19,817) (16,029)
Fair Value    
Less than 12 months   58,065
12 months or more 366,465 353,017
Total 366,465 411,082
Unrealized Losses    
Less than 12 months   (8,009)
12 months or more (93,401) (55,729)
Total (93,401) (63,738)
Residential mortgage-backed securities    
Fair Value    
Less than 12 months 431 366,221
12 months or more 1,088,344 876,668
Total 1,088,775 1,242,889
Unrealized Losses    
Less than 12 months (10) (38,111)
Unrealized Losses (254,518) (182,606)
Total (254,528) (220,717)
Fair Value    
Less than 12 months   0
12 months or more 335,211 374,213
Total 335,211 374,213
Unrealized Losses    
Less than 12 months   0
12 months or more (87,537) (69,279)
Total (87,537) (69,279)
Corporate debt securities    
Fair Value    
Less than 12 months 1,457 39,037
12 months or more 219,074 204,310
Total 220,531 243,347
Unrealized Losses    
Less than 12 months (2) (5,079)
Unrealized Losses (23,732) (19,556)
Total $ (23,734) $ (24,635)