Investments - Investments Held in a Loss Position (Details) - USD ($) $ in Thousands	Dec. 31, 2024	Dec. 31, 2023
Fair value
Fair value, total	$ 2,696,054	$ 2,934,215
Fair value, less than 12 months	543,892	293,116
Fair value, 12 months or longer	2,152,162	2,641,099
Unrealized Loss
Unrealized loss, total	227,504	274,690
Unrealized loss, less than 12 months	18,050	21,226
Unrealized loss, 12 months or longer	209,454	253,464
U.S. Treasury obligations
Fair value
Fair value, total	213,505	213,634
Fair value, less than 12 months	33,822	32,925
Fair value, 12 months or longer	179,683	180,709
Unrealized Loss
Unrealized loss, total	13,143	16,474
Unrealized loss, less than 12 months	1,561	1,364
Unrealized loss, 12 months or longer	11,582	15,110
U.S. Government-sponsored enterprise obligations
Fair value
Fair value, total	14,894	18,428
Fair value, less than 12 months	2,603	4,128
Fair value, 12 months or longer	12,291	14,300
Unrealized Loss
Unrealized loss, total	692	1,030
Unrealized loss, less than 12 months	69	242
Unrealized loss, 12 months or longer	623	788
State and municipal bonds
Fair value
Fair value, total	378,425	378,313
Fair value, less than 12 months	82,312	48,960
Fair value, 12 months or longer	296,113	329,353
Unrealized Loss
Unrealized loss, total	25,508	29,871
Unrealized loss, less than 12 months	2,261	2,287
Unrealized loss, 12 months or longer	23,247	27,584
Corporate debt
Fair value
Fair value, total	1,350,139	1,524,940
Fair value, less than 12 months	222,614	84,221
Fair value, 12 months or longer	1,127,525	1,440,719
Unrealized Loss
Unrealized loss, total	107,065	136,759
Unrealized loss, less than 12 months	9,593	5,054
Unrealized loss, 12 months or longer	97,472	131,705
Residential mortgage-backed securities
Fair value
Fair value, total	378,461	313,082
Fair value, less than 12 months	118,908	74,463
Fair value, 12 months or longer	259,553	238,619
Unrealized Loss
Unrealized loss, total	58,826	53,795
Unrealized loss, less than 12 months	3,810	10,271
Unrealized loss, 12 months or longer	55,016	43,524
Agency commercial mortgage-backed securities
Fair value
Fair value, total	6,727	7,955
Fair value, less than 12 months	426	212
Fair value, 12 months or longer	6,301	7,743
Unrealized Loss
Unrealized loss, total	873	987
Unrealized loss, less than 12 months	8	1
Unrealized loss, 12 months or longer	865	986
Other commercial mortgage-backed securities
Fair value
Fair value, total	151,386	184,416
Fair value, less than 12 months	17,337	18,092
Fair value, 12 months or longer	134,049	166,324
Unrealized Loss
Unrealized loss, total	12,266	20,904
Unrealized loss, less than 12 months	188	1,140
Unrealized loss, 12 months or longer	12,078	19,764
Other asset-backed securities
Fair value
Fair value, total	202,517	293,447
Fair value, less than 12 months	65,870	30,115
Fair value, 12 months or longer	136,647	263,332
Unrealized Loss
Unrealized loss, total	9,131	14,870
Unrealized loss, less than 12 months	560	867
Unrealized loss, 12 months or longer	$ 8,571	$ 14,003