Investments (Investments Held in a Loss Position) (Details) - USD ($) $ in Thousands	Mar. 31, 2024	Dec. 31, 2023
Fair value
Fair Value	$ 2,942,293	$ 2,934,215
Less than 12 months, Fair Value	958,097	293,116
12 months or longer, Fair Value	1,984,196	2,641,099
Unrealized Loss
Unrealized Loss	275,926	274,690
Less than 12 months, Unrealized Loss	75,300	21,226
12 months or longer, Unrealized Loss	200,626	253,464
U.S. Treasury obligations
Fair value
Fair Value	226,103	213,634
Less than 12 months, Fair Value	40,747	32,925
12 months or longer, Fair Value	185,356	180,709
Unrealized Loss
Unrealized Loss	17,597	16,474
Less than 12 months, Unrealized Loss	1,412	1,364
12 months or longer, Unrealized Loss	16,185	15,110
U.S. Government-sponsored enterprise obligations
Fair value
Fair Value	18,928	18,428
Less than 12 months, Fair Value	10,631	4,128
12 months or longer, Fair Value	8,297	14,300
Unrealized Loss
Unrealized Loss	1,030	1,030
Less than 12 months, Unrealized Loss	486	242
12 months or longer, Unrealized Loss	544	788
State and municipal bonds
Fair value
Fair Value	389,539	378,313
Less than 12 months, Fair Value	98,395	48,960
12 months or longer, Fair Value	291,144	329,353
Unrealized Loss
Unrealized Loss	30,020	29,871
Less than 12 months, Unrealized Loss	5,941	2,287
12 months or longer, Unrealized Loss	24,079	27,584
Corporate debt
Fair value
Fair Value	1,498,062	1,524,940
Less than 12 months, Fair Value	459,157	84,221
12 months or longer, Fair Value	1,038,905	1,440,719
Unrealized Loss
Unrealized Loss	136,791	136,759
Less than 12 months, Unrealized Loss	38,451	5,054
12 months or longer, Unrealized Loss	98,340	131,705
Residential mortgage-backed securities
Fair value
Fair Value	335,297	313,082
Less than 12 months, Fair Value	130,058	74,463
12 months or longer, Fair Value	205,239	238,619
Unrealized Loss
Unrealized Loss	59,073	53,795
Less than 12 months, Unrealized Loss	16,800	10,271
12 months or longer, Unrealized Loss	42,273	43,524
Agency commercial mortgage-backed securities
Fair value
Fair Value	7,950	7,955
Less than 12 months, Fair Value	2,652	212
12 months or longer, Fair Value	5,298	7,743
Unrealized Loss
Unrealized Loss	1,014	987
Less than 12 months, Unrealized Loss	344	1
12 months or longer, Unrealized Loss	670	986
Other commercial mortgage-backed securities
Fair value
Fair Value	214,030	184,416
Less than 12 months, Fair Value	90,586	18,092
12 months or longer, Fair Value	123,444	166,324
Unrealized Loss
Unrealized Loss	19,948	20,904
Less than 12 months, Unrealized Loss	8,120	1,140
12 months or longer, Unrealized Loss	11,828	19,764
Other asset-backed securities
Fair value
Fair Value	252,384	293,447
Less than 12 months, Fair Value	125,871	30,115
12 months or longer, Fair Value	126,513	263,332
Unrealized Loss
Unrealized Loss	10,453	14,870
Less than 12 months, Unrealized Loss	3,746	867
12 months or longer, Unrealized Loss	$ 6,707	$ 14,003