Investments in Fixed Maturity Securities (Investments in Fixed Maturity Securities) (Details) - USD ($) $ in Thousands	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Schedule of Available-for-sale and Held-to-maturity Securities [Table] [Line Items]
Total held to maturity, Amortized Cost	$ 53,415	$ 51,410
Held-to-maturity, allowance for credit loss	(43)	(114)	$ (387)
Total held to maturity, Gross Unrealized Gains	3,239	4,277
Total held to maturity, Gross Unrealized Losses	0	0
Total held to maturity, Fair Value	56,611	55,573
Total held to maturity, Carrying Value	53,372	51,296
Fixed maturity securities available for sale at cost	20,861,830	18,664,073
Cumulative effect adjustment resulting from changes in accounting principles	(36,708)	(37,352)	(22,238)
Total available for sale, Gross Unrealized Gains	110,989	32,166
Total available for sale, Gross Unrealized Losses	(811,175)	(1,122,834)
Total available for sale, Fair Value	20,124,936	17,536,053
Total available for sale, Carrying Value	20,124,936	17,536,053
Fixed maturity securities, amortized cost	20,915,245	18,715,483
Total investments in fixed maturity securities, Gross Unrealized Gains	114,228	36,443
Total investments in fixed maturity securities, Gross Unrealized Losses	(811,175)	(1,122,834)
Total investments in fixed maturity securities, Fair Value	20,181,547	17,591,626
Total investments in fixed maturity securities, carrying value	20,178,308	17,587,349
Fixed maturity securities, allowance for credit loss	(36,751)	(37,466)
State and municipal
Schedule of Available-for-sale and Held-to-maturity Securities [Table] [Line Items]
Total held to maturity, Amortized Cost	50,547	47,802
Held-to-maturity, allowance for credit loss	(43)	(114)
Total held to maturity, Gross Unrealized Gains	3,132	4,239
Total held to maturity, Gross Unrealized Losses	0	0
Total held to maturity, Fair Value	53,636	51,927
Total held to maturity, Carrying Value	50,504	47,688
Fixed maturity securities available for sale at cost	2,742,922	3,046,335
Cumulative effect adjustment resulting from changes in accounting principles	(757)	0
Total available for sale, Gross Unrealized Gains	14,719	12,355
Total available for sale, Gross Unrealized Losses	(122,462)	(168,592)
Total available for sale, Fair Value	2,634,422	2,890,098
Total available for sale, Carrying Value	2,634,422	2,890,098
Residential mortgage-backed
Schedule of Available-for-sale and Held-to-maturity Securities [Table] [Line Items]
Total held to maturity, Amortized Cost	2,868	3,608
Held-to-maturity, allowance for credit loss	0	0
Total held to maturity, Gross Unrealized Gains	107	38
Total held to maturity, Gross Unrealized Losses	0	0
Total held to maturity, Fair Value	2,975	3,646
Total held to maturity, Carrying Value	2,868	3,608
Fixed maturity securities available for sale at cost	1,773,206	1,308,019
Cumulative effect adjustment resulting from changes in accounting principles	0	(18)
Total available for sale, Gross Unrealized Gains	12,780	395
Total available for sale, Gross Unrealized Losses	(163,844)	(171,595)
Total available for sale, Fair Value	1,622,142	1,136,801
Total available for sale, Carrying Value	1,622,142	1,136,801
U.S. government and government agency
Schedule of Available-for-sale and Held-to-maturity Securities [Table] [Line Items]
Fixed maturity securities available for sale at cost	1,762,997	960,479
Cumulative effect adjustment resulting from changes in accounting principles	0	0
Total available for sale, Gross Unrealized Gains	11,403	937
Total available for sale, Gross Unrealized Losses	(57,669)	(69,158)
Total available for sale, Fair Value	1,716,731	892,258
Total available for sale, Carrying Value	1,716,731	892,258
Special Revenue
Schedule of Available-for-sale and Held-to-maturity Securities [Table] [Line Items]
Fixed maturity securities available for sale at cost	1,682,550	1,837,309
Cumulative effect adjustment resulting from changes in accounting principles	0	0
Total available for sale, Gross Unrealized Gains	5,651	3,662
Total available for sale, Gross Unrealized Losses	(82,006)	(119,474)
Total available for sale, Fair Value	1,606,195	1,721,497
Total available for sale, Carrying Value	1,606,195	1,721,497
Debt, State Government
Schedule of Available-for-sale and Held-to-maturity Securities [Table] [Line Items]
Fixed maturity securities available for sale at cost	394,429	387,709
Cumulative effect adjustment resulting from changes in accounting principles	0	0
Total available for sale, Gross Unrealized Gains	3,550	2,651
Total available for sale, Gross Unrealized Losses	(16,405)	(21,335)
Total available for sale, Fair Value	381,574	369,025
Total available for sale, Carrying Value	381,574	369,025
Revenue Subject to Refund
Schedule of Available-for-sale and Held-to-maturity Securities [Table] [Line Items]
Fixed maturity securities available for sale at cost	103,029	156,106
Cumulative effect adjustment resulting from changes in accounting principles	0	0
Total available for sale, Gross Unrealized Gains	1,634	2,741
Total available for sale, Gross Unrealized Losses	(185)	(7)
Total available for sale, Fair Value	104,478	158,840
Total available for sale, Carrying Value	104,478	158,840
Corporation
Schedule of Available-for-sale and Held-to-maturity Securities [Table] [Line Items]
Fixed maturity securities available for sale at cost	166,873	210,228
Cumulative effect adjustment resulting from changes in accounting principles	(757)	0
Total available for sale, Gross Unrealized Gains	696	334
Total available for sale, Gross Unrealized Losses	(11,973)	(10,923)
Total available for sale, Fair Value	154,839	199,639
Total available for sale, Carrying Value	154,839	199,639
Debt, Local Government
Schedule of Available-for-sale and Held-to-maturity Securities [Table] [Line Items]
Fixed maturity securities available for sale at cost	396,041	454,983
Cumulative effect adjustment resulting from changes in accounting principles	0	0
Total available for sale, Gross Unrealized Gains	3,188	2,967
Total available for sale, Gross Unrealized Losses	(11,893)	(16,853)
Total available for sale, Fair Value	387,336	441,097
Total available for sale, Carrying Value	387,336	441,097
Commercial Mortgage Backed Securities
Schedule of Available-for-sale and Held-to-maturity Securities [Table] [Line Items]
Fixed maturity securities available for sale at cost	657,157	547,757
Cumulative effect adjustment resulting from changes in accounting principles	(158)	0
Total available for sale, Gross Unrealized Gains	626	215
Total available for sale, Gross Unrealized Losses	(13,312)	(19,363)
Total available for sale, Fair Value	644,313	528,609
Total available for sale, Carrying Value	644,313	528,609
Mortgage- Backed
Schedule of Available-for-sale and Held-to-maturity Securities [Table] [Line Items]
Fixed maturity securities available for sale at cost	2,430,363	1,855,776
Cumulative effect adjustment resulting from changes in accounting principles	(158)	(18)	0
Total available for sale, Gross Unrealized Gains	13,406	610
Total available for sale, Gross Unrealized Losses	(177,156)	(190,958)
Total available for sale, Fair Value	2,266,455	1,665,410
Total available for sale, Carrying Value	2,266,455	1,665,410
Asset-backed Securities
Schedule of Available-for-sale and Held-to-maturity Securities [Table] [Line Items]
Fixed maturity securities available for sale at cost	4,252,883	4,132,365
Cumulative effect adjustment resulting from changes in accounting principles	(1,164)	0
Total available for sale, Gross Unrealized Gains	8,527	2,730
Total available for sale, Gross Unrealized Losses	(73,206)	(152,322)
Total available for sale, Fair Value	4,187,040	3,982,773
Total available for sale, Carrying Value	4,187,040	3,982,773
Industrial Property
Schedule of Available-for-sale and Held-to-maturity Securities [Table] [Line Items]
Fixed maturity securities available for sale at cost	3,679,219	3,491,645
Cumulative effect adjustment resulting from changes in accounting principles	(40)	(1,704)
Total available for sale, Gross Unrealized Gains	24,312	4,439
Total available for sale, Gross Unrealized Losses	(143,936)	(241,381)
Total available for sale, Fair Value	3,559,555	3,252,999
Total available for sale, Carrying Value	3,559,555	3,252,999
Debt Security, Corporate, US
Schedule of Available-for-sale and Held-to-maturity Securities [Table] [Line Items]
Fixed maturity securities available for sale at cost	2,838,220	2,585,247
Cumulative effect adjustment resulting from changes in accounting principles	(4,986)	(2,997)
Total available for sale, Gross Unrealized Gains	14,681	5,505
Total available for sale, Gross Unrealized Losses	(68,681)	(117,383)
Total available for sale, Fair Value	2,779,234	2,470,372
Total available for sale, Carrying Value	2,779,234	2,470,372
Public Utility, Bonds
Schedule of Available-for-sale and Held-to-maturity Securities [Table] [Line Items]
Fixed maturity securities available for sale at cost	701,865	586,066
Cumulative effect adjustment resulting from changes in accounting principles	0	0
Total available for sale, Gross Unrealized Gains	6,471	1,307
Total available for sale, Gross Unrealized Losses	(23,412)	(36,325)
Total available for sale, Fair Value	684,924	551,048
Total available for sale, Carrying Value	684,924	551,048
Other Security Investments
Schedule of Available-for-sale and Held-to-maturity Securities [Table] [Line Items]
Fixed maturity securities available for sale at cost	635,975	441,230
Cumulative effect adjustment resulting from changes in accounting principles	0	0
Total available for sale, Gross Unrealized Gains	1,605	0
Total available for sale, Gross Unrealized Losses	(7,234)	(11,657)
Total available for sale, Fair Value	630,346	429,573
Total available for sale, Carrying Value	630,346	429,573
Corporate
Schedule of Available-for-sale and Held-to-maturity Securities [Table] [Line Items]
Fixed maturity securities available for sale at cost	7,855,279	7,104,188
Cumulative effect adjustment resulting from changes in accounting principles	(5,026)	(4,701)	(16)
Total available for sale, Gross Unrealized Gains	47,069	11,251
Total available for sale, Gross Unrealized Losses	(243,263)	(406,746)
Total available for sale, Fair Value	7,654,059	6,703,992
Total available for sale, Carrying Value	7,654,059	6,703,992
Foreign government
Schedule of Available-for-sale and Held-to-maturity Securities [Table] [Line Items]
Fixed maturity securities available for sale at cost	1,817,386	1,564,930
Cumulative effect adjustment resulting from changes in accounting principles	(29,603)	(32,633)	$ (22,222)
Total available for sale, Gross Unrealized Gains	15,865	4,283
Total available for sale, Gross Unrealized Losses	(137,419)	(135,058)
Total available for sale, Fair Value	1,666,229	1,401,522
Total available for sale, Carrying Value	$ 1,666,229	$ 1,401,522