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Debt Securities - Debt securities in an unrealized loss position (Details) - USD ($)
$ in Thousands
Mar. 31, 2023
Dec. 31, 2022
Fair Value    
Less than 12 months $ 237,884 $ 1,070,674
12 months or more 2,102,068 1,368,859
Total 2,339,952 2,439,533
Unrealized Losses    
Less than 12 months (8,637) (81,841)
Unrealized Losses (271,946) (229,363)
Total (280,583) (311,204)
Fair Value    
Less than 12 months   58,065
12 months or more 780,653 727,230
Total 780,653 785,295
Unrealized Losses    
Less than 12 months   (8,009)
12 months or more (126,906) (125,008)
Total (126,906) (133,017)
U.S. Treasury securities    
Fair Value    
Less than 12 months 964 74
12 months or more 83,668 113,987
Total 84,632 114,061
Unrealized Losses    
Less than 12 months (32) 0
Unrealized Losses (2,518) (3,744)
Total (2,550) (3,744)
Obligations of U.S. government corporations and agencies    
Fair Value    
Less than 12 months 11,262 19,603
12 months or more 196 0
Total 11,458 19,603
Unrealized Losses    
Less than 12 months (223) (321)
Unrealized Losses (8) 0
Total (231) (321)
Obligations of states and political subdivisions    
Fair Value    
Less than 12 months 80,541 166,147
12 months or more 133,343 75,217
Total 213,884 241,364
Unrealized Losses    
Less than 12 months (755) (10,059)
Unrealized Losses (20,035) (16,016)
Total (20,790) (26,075)
Asset-backed securities    
Fair Value    
Less than 12 months 16,954 390,164
12 months or more 452,193 79,711
Total 469,147 469,875
Unrealized Losses    
Less than 12 months (346) (15,648)
Unrealized Losses (18,741) (4,035)
Total (19,087) (19,683)
Commercial mortgage-backed securities    
Fair Value    
Less than 12 months 7,634 89,428
12 months or more 101,193 18,966
Total 108,827 108,394
Unrealized Losses    
Less than 12 months (290) (12,623)
Unrealized Losses (14,603) (3,406)
Total (14,893) (16,029)
Fair Value    
Less than 12 months   58,065
12 months or more 406,940 353,017
Total 406,940 411,082
Unrealized Losses    
Less than 12 months   (8,009)
12 months or more (63,198) (55,729)
Total (63,198) (63,738)
Residential mortgage-backed securities    
Fair Value    
Less than 12 months 103,167 366,221
12 months or more 1,122,195 876,668
Total 1,225,362 1,242,889
Unrealized Losses    
Less than 12 months (5,778) (38,111)
Unrealized Losses (194,821) (182,606)
Total (200,599) (220,717)
Fair Value    
Less than 12 months   0
12 months or more 373,713 374,213
Total 373,713 374,213
Unrealized Losses    
Less than 12 months   0
12 months or more (63,708) (69,279)
Total (63,708) (69,279)
Corporate debt securities    
Fair Value    
Less than 12 months 17,362 39,037
12 months or more 209,280 204,310
Total 226,642 243,347
Unrealized Losses    
Less than 12 months (1,213) (5,079)
Unrealized Losses (21,220) (19,556)
Total $ (22,433) $ (24,635)