Investments in Fixed Maturity Securities (Investments in Fixed Maturity Securities) (Details) - USD ($) $ in Thousands	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Schedule of Available-for-sale and Held-to-maturity Securities [Table] [Line Items]
Total held to maturity, Amortized Cost	$ 51,410	$ 74,368
Held-to-maturity, allowance for credit loss	(114)	(387)	$ (798)
Total held to maturity, Gross Unrealized Gains	4,277	11,445
Total held to maturity, Gross Unrealized Losses	0	0
Total held to maturity, Fair Value	55,573	85,426
Total held to maturity, Carrying Value	51,296	73,981
Fixed maturity securities available for sale at cost	18,664,073	16,396,936
Cumulative effect adjustment resulting from changes in accounting principles	(37,352)	(22,238)	(1,782)
Total available for sale, Gross Unrealized Gains	32,166	278,207
Total available for sale, Gross Unrealized Losses	(1,122,834)	(124,213)
Total available for sale, Fair Value	17,536,053	16,528,692
Total available for sale, Carrying Value	17,536,053	16,528,692
Fixed maturity securities, amortized cost	18,715,483	16,471,304
Total investments in fixed maturity securities, Gross Unrealized Gains	36,443	289,652
Total investments in fixed maturity securities, Gross Unrealized Losses	(1,122,834)	(124,213)
Total investments in fixed maturity securities, Fair Value	17,591,626	16,614,118
Total investments in fixed maturity securities, carrying value	17,587,349	16,602,673
Fixed maturity securities, allowance for credit loss	(37,466)	(22,625)
State and municipal
Schedule of Available-for-sale and Held-to-maturity Securities [Table] [Line Items]
Total held to maturity, Amortized Cost	47,802	69,539
Held-to-maturity, allowance for credit loss	(114)	(387)
Total held to maturity, Gross Unrealized Gains	4,239	10,813
Total held to maturity, Gross Unrealized Losses	0	0
Total held to maturity, Fair Value	51,927	79,965
Total held to maturity, Carrying Value	47,688	69,152
Fixed maturity securities available for sale at cost	3,046,335	3,176,042
Cumulative effect adjustment resulting from changes in accounting principles	0	0
Total available for sale, Gross Unrealized Gains	12,355	137,650
Total available for sale, Gross Unrealized Losses	(168,592)	(9,559)
Total available for sale, Fair Value	2,890,098	3,304,133
Total available for sale, Carrying Value	2,890,098	3,304,133
Residential mortgage-backed
Schedule of Available-for-sale and Held-to-maturity Securities [Table] [Line Items]
Total held to maturity, Amortized Cost	3,608	4,829
Held-to-maturity, allowance for credit loss	0	0
Total held to maturity, Gross Unrealized Gains	38	632
Total held to maturity, Gross Unrealized Losses	0	0
Total held to maturity, Fair Value	3,646	5,461
Total held to maturity, Carrying Value	3,608	4,829
Fixed maturity securities available for sale at cost	1,308,019	940,744
Cumulative effect adjustment resulting from changes in accounting principles	(18)	0
Total available for sale, Gross Unrealized Gains	395	9,896
Total available for sale, Gross Unrealized Losses	(171,595)	(11,321)
Total available for sale, Fair Value	1,136,801	939,319
Total available for sale, Carrying Value	1,136,801	939,319
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	960,479	851,128
Cumulative effect adjustment resulting from changes in accounting principles	0	0
Total available for sale, Gross Unrealized Gains	937	8,509
Total available for sale, Gross Unrealized Losses	(69,158)	(4,294)
Total available for sale, Fair Value	892,258	855,343
Total available for sale, Carrying Value	892,258	855,343
Special Revenue
Schedule of Available-for-sale and Held-to-maturity Securities [Table] [Line Items]
Fixed maturity securities available for sale at cost	1,837,309	2,016,382
Cumulative effect adjustment resulting from changes in accounting principles	0	0
Total available for sale, Gross Unrealized Gains	3,662	62,961
Total available for sale, Gross Unrealized Losses	(119,474)	(5,706)
Total available for sale, Fair Value	1,721,497	2,073,637
Total available for sale, Carrying Value	1,721,497	2,073,637
Debt, State Government
Schedule of Available-for-sale and Held-to-maturity Securities [Table] [Line Items]
Fixed maturity securities available for sale at cost	387,709	388,110
Cumulative effect adjustment resulting from changes in accounting principles	0	0
Total available for sale, Gross Unrealized Gains	2,651	23,152
Total available for sale, Gross Unrealized Losses	(21,335)	(1,015)
Total available for sale, Fair Value	369,025	410,247
Total available for sale, Carrying Value	369,025	410,247
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	156,106	202,633
Cumulative effect adjustment resulting from changes in accounting principles	0	0
Total available for sale, Gross Unrealized Gains	2,741	14,891
Total available for sale, Gross Unrealized Losses	(7)	(574)
Total available for sale, Fair Value	158,840	216,950
Total available for sale, Carrying Value	158,840	216,950
Corporation
Schedule of Available-for-sale and Held-to-maturity Securities [Table] [Line Items]
Fixed maturity securities available for sale at cost	210,228	166,943
Cumulative effect adjustment resulting from changes in accounting principles	0	0
Total available for sale, Gross Unrealized Gains	334	7,191
Total available for sale, Gross Unrealized Losses	(10,923)	(1,532)
Total available for sale, Fair Value	199,639	172,602
Total available for sale, Carrying Value	199,639	172,602
Debt, Local Government
Schedule of Available-for-sale and Held-to-maturity Securities [Table] [Line Items]
Fixed maturity securities available for sale at cost	454,983	401,974
Cumulative effect adjustment resulting from changes in accounting principles	0	0
Total available for sale, Gross Unrealized Gains	2,967	29,455
Total available for sale, Gross Unrealized Losses	(16,853)	(732)
Total available for sale, Fair Value	441,097	430,697
Total available for sale, Carrying Value	441,097	430,697
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	547,757	125,709
Cumulative effect adjustment resulting from changes in accounting principles	0	0
Total available for sale, Gross Unrealized Gains	215	3,388
Total available for sale, Gross Unrealized Losses	(19,363)	(341)
Total available for sale, Fair Value	528,609	128,756
Total available for sale, Carrying Value	528,609	128,756
Mortgage- Backed
Schedule of Available-for-sale and Held-to-maturity Securities [Table] [Line Items]
Fixed maturity securities available for sale at cost	1,855,776	1,066,453
Cumulative effect adjustment resulting from changes in accounting principles	(18)	0
Total available for sale, Gross Unrealized Gains	610	13,284
Total available for sale, Gross Unrealized Losses	(190,958)	(11,662)
Total available for sale, Fair Value	1,665,410	1,068,075
Total available for sale, Carrying Value	1,665,410	1,068,075
Asset-backed Securities
Schedule of Available-for-sale and Held-to-maturity Securities [Table] [Line Items]
Fixed maturity securities available for sale at cost	4,132,365	4,504,950
Cumulative effect adjustment resulting from changes in accounting principles	0	0
Total available for sale, Gross Unrealized Gains	2,730	4,409
Total available for sale, Gross Unrealized Losses	(152,322)	(18,794)
Total available for sale, Fair Value	3,982,773	4,490,565
Total available for sale, Carrying Value	3,982,773	4,490,565
Industrial Property
Schedule of Available-for-sale and Held-to-maturity Securities [Table] [Line Items]
Fixed maturity securities available for sale at cost	3,491,645	3,231,520
Cumulative effect adjustment resulting from changes in accounting principles	(1,704)	(16)
Total available for sale, Gross Unrealized Gains	4,439	62,751
Total available for sale, Gross Unrealized Losses	(241,381)	(21,092)
Total available for sale, Fair Value	3,252,999	3,273,163
Total available for sale, Carrying Value	3,252,999	3,273,163
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	2,585,247	1,739,282
Cumulative effect adjustment resulting from changes in accounting principles	(2,997)	0
Total available for sale, Gross Unrealized Gains	5,505	30,709
Total available for sale, Gross Unrealized Losses	(117,383)	(6,591)
Total available for sale, Fair Value	2,470,372	1,763,400
Total available for sale, Carrying Value	2,470,372	1,763,400
Public Utility, Bonds
Schedule of Available-for-sale and Held-to-maturity Securities [Table] [Line Items]
Fixed maturity securities available for sale at cost	586,066	396,242
Cumulative effect adjustment resulting from changes in accounting principles	0	0
Total available for sale, Gross Unrealized Gains	1,307	13,262
Total available for sale, Gross Unrealized Losses	(36,325)	(3,202)
Total available for sale, Fair Value	551,048	406,302
Total available for sale, Carrying Value	551,048	406,302
Other Security Investments
Schedule of Available-for-sale and Held-to-maturity Securities [Table] [Line Items]
Fixed maturity securities available for sale at cost	441,230	154,210
Cumulative effect adjustment resulting from changes in accounting principles	0	0
Total available for sale, Gross Unrealized Gains	0	125
Total available for sale, Gross Unrealized Losses	(11,657)	(1,525)
Total available for sale, Fair Value	429,573	152,810
Total available for sale, Carrying Value	429,573	152,810
Corporate
Schedule of Available-for-sale and Held-to-maturity Securities [Table] [Line Items]
Fixed maturity securities available for sale at cost	7,104,188	5,521,254
Cumulative effect adjustment resulting from changes in accounting principles	(4,701)	(16)	(518)
Total available for sale, Gross Unrealized Gains	11,251	106,847
Total available for sale, Gross Unrealized Losses	(406,746)	(32,410)
Total available for sale, Fair Value	6,703,992	5,595,675
Total available for sale, Carrying Value	6,703,992	5,595,675
Foreign government
Schedule of Available-for-sale and Held-to-maturity Securities [Table] [Line Items]
Fixed maturity securities available for sale at cost	1,564,930	1,277,109
Cumulative effect adjustment resulting from changes in accounting principles	(32,633)	(22,222)	$ (1,264)
Total available for sale, Gross Unrealized Gains	4,283	7,508
Total available for sale, Gross Unrealized Losses	(135,058)	(47,494)
Total available for sale, Fair Value	1,401,522	1,214,901
Total available for sale, Carrying Value	$ 1,401,522	$ 1,214,901