Investments (Investments Held in a Loss Position) (Details) - USD ($) $ in Thousands	Sep. 30, 2025	Dec. 31, 2024
Fair value
Fair Value	$ 2,131,830	$ 2,696,054
Less than 12 months, Fair Value	348,142	543,892
12 months or longer, Fair Value	1,783,688	2,152,162
Unrealized Loss
Unrealized Loss	144,827	227,504
Less than 12 months, Unrealized Loss	19,446	18,050
12 months or longer, Unrealized Loss	125,381	209,454
U.S. Treasury obligations
Fair value
Fair Value	190,822	213,505
Less than 12 months, Fair Value	13,661	33,822
12 months or longer, Fair Value	177,161	179,683
Unrealized Loss
Unrealized Loss	6,799	13,143
Less than 12 months, Unrealized Loss	972	1,561
12 months or longer, Unrealized Loss	5,827	11,582
U.S. Government-sponsored enterprise obligations
Fair value
Fair Value	8,996	14,894
Less than 12 months, Fair Value	0	2,603
12 months or longer, Fair Value	8,996	12,291
Unrealized Loss
Unrealized Loss	342	692
Less than 12 months, Unrealized Loss	0	69
12 months or longer, Unrealized Loss	342	623
State and municipal bonds
Fair value
Fair Value	288,050	378,425
Less than 12 months, Fair Value	40,373	82,312
12 months or longer, Fair Value	247,677	296,113
Unrealized Loss
Unrealized Loss	15,035	25,508
Less than 12 months, Unrealized Loss	1,840	2,261
12 months or longer, Unrealized Loss	13,195	23,247
Corporate debt
Fair value
Fair Value	1,069,619	1,350,139
Less than 12 months, Fair Value	154,904	222,614
12 months or longer, Fair Value	914,715	1,127,525
Unrealized Loss
Unrealized Loss	61,437	107,065
Less than 12 months, Unrealized Loss	10,057	9,593
12 months or longer, Unrealized Loss	51,380	97,472
Residential mortgage-backed securities
Fair value
Fair Value	315,729	378,461
Less than 12 months, Fair Value	76,550	118,908
12 months or longer, Fair Value	239,179	259,553
Unrealized Loss
Unrealized Loss	45,996	58,826
Less than 12 months, Unrealized Loss	4,951	3,810
12 months or longer, Unrealized Loss	41,045	55,016
Agency commercial mortgage-backed securities
Fair value
Fair Value	4,781	6,727
Less than 12 months, Fair Value	800	426
12 months or longer, Fair Value	3,981	6,301
Unrealized Loss
Unrealized Loss	720	873
Less than 12 months, Unrealized Loss	122	8
12 months or longer, Unrealized Loss	598	865
Other commercial mortgage-backed securities
Fair value
Fair Value	124,738	151,386
Less than 12 months, Fair Value	25,528	17,337
12 months or longer, Fair Value	99,210	134,049
Unrealized Loss
Unrealized Loss	7,763	12,266
Less than 12 months, Unrealized Loss	325	188
12 months or longer, Unrealized Loss	7,438	12,078
Other asset-backed securities
Fair value
Fair Value	129,095	202,517
Less than 12 months, Fair Value	36,326	65,870
12 months or longer, Fair Value	92,769	136,647
Unrealized Loss
Unrealized Loss	6,735	9,131
Less than 12 months, Unrealized Loss	1,179	560
12 months or longer, Unrealized Loss	$ 5,556	$ 8,571