Investments in Fixed Maturity Securities (Investments in Fixed Maturity Securities) (Details) - USD ($) $ in Thousands	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Schedule of Available-for-sale and Held-to-maturity Securities [Table] [Line Items]
Total held to maturity, Amortized Cost	$ 74,368	$ 73,572
Held-to-maturity, allowance for credit loss	(387)	(798)	$ 0
Total held to maturity, Gross Unrealized Gains	11,445	14,260
Total held to maturity, Gross Unrealized Losses	0	0
Total held to maturity, Fair Value	85,426	87,034
Total held to maturity, Carrying Value	73,981	72,774
Fixed maturity securities available for sale at cost	16,396,936	13,682,286
Cumulative effect adjustment resulting from changes in accounting principles	(22,238)	(1,782)	0
Total available for sale, Gross Unrealized Gains	278,207	500,283
Total available for sale, Gross Unrealized Losses	(124,213)	(94,192)
Total available for sale, Fair Value	16,528,692	14,086,595
Total available for sale, Carrying Value	16,528,692	14,086,595
Fixed maturity securities, amortized cost	16,471,304	13,755,858
Total investments in fixed maturity securities, Gross Unrealized Gains	289,652	514,543
Total investments in fixed maturity securities, Gross Unrealized Losses	(124,213)	(94,192)
Total investments in fixed maturity securities, Fair Value	16,614,118	14,173,629
Total investments in fixed maturity securities, carrying value	16,602,673	14,159,369
Fixed maturity securities, allowance for credit loss	(22,625)	(2,580)
State and municipal
Schedule of Available-for-sale and Held-to-maturity Securities [Table] [Line Items]
Total held to maturity, Amortized Cost	69,539	67,117
Held-to-maturity, allowance for credit loss	(387)	(798)
Total held to maturity, Gross Unrealized Gains	10,813	13,217
Total held to maturity, Gross Unrealized Losses	0	0
Total held to maturity, Fair Value	79,965	79,536
Total held to maturity, Carrying Value	69,152	66,319
Fixed maturity securities available for sale at cost	3,176,042	3,421,579
Cumulative effect adjustment resulting from changes in accounting principles	0	0
Total available for sale, Gross Unrealized Gains	137,650	201,154
Total available for sale, Gross Unrealized Losses	(9,559)	(2,069)
Total available for sale, Fair Value	3,304,133	3,620,664
Total available for sale, Carrying Value	3,304,133	3,620,664
Residential mortgage-backed
Schedule of Available-for-sale and Held-to-maturity Securities [Table] [Line Items]
Total held to maturity, Amortized Cost	4,829	6,455
Held-to-maturity, allowance for credit loss	0	0
Total held to maturity, Gross Unrealized Gains	632	1,043
Total held to maturity, Gross Unrealized Losses	0	0
Total held to maturity, Fair Value	5,461	7,498
Total held to maturity, Carrying Value	4,829	6,455
Fixed maturity securities available for sale at cost	940,744	813,187
Cumulative effect adjustment resulting from changes in accounting principles	0	0
Total available for sale, Gross Unrealized Gains	9,896	24,664
Total available for sale, Gross Unrealized Losses	(11,321)	(5,238)
Total available for sale, Fair Value	939,319	832,613
Total available for sale, Carrying Value	939,319	832,613
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	851,128	586,020
Cumulative effect adjustment resulting from changes in accounting principles	0	0
Total available for sale, Gross Unrealized Gains	8,509	18,198
Total available for sale, Gross Unrealized Losses	(4,294)	(347)
Total available for sale, Fair Value	855,343	603,871
Total available for sale, Carrying Value	855,343	603,871
Special Revenue
Schedule of Available-for-sale and Held-to-maturity Securities [Table] [Line Items]
Fixed maturity securities available for sale at cost	2,016,382	2,137,162
Cumulative effect adjustment resulting from changes in accounting principles	0	0
Total available for sale, Gross Unrealized Gains	62,961	96,924
Total available for sale, Gross Unrealized Losses	(5,706)	(714)
Total available for sale, Fair Value	2,073,637	2,233,372
Total available for sale, Carrying Value	2,073,637	2,233,372
Debt, State Government
Schedule of Available-for-sale and Held-to-maturity Securities [Table] [Line Items]
Fixed maturity securities available for sale at cost	388,110	417,397
Cumulative effect adjustment resulting from changes in accounting principles	0	0
Total available for sale, Gross Unrealized Gains	23,152	33,407
Total available for sale, Gross Unrealized Losses	(1,015)	0
Total available for sale, Fair Value	410,247	450,804
Total available for sale, Carrying Value	410,247	450,804
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	202,633	250,081
Cumulative effect adjustment resulting from changes in accounting principles	0	0
Total available for sale, Gross Unrealized Gains	14,891	21,472
Total available for sale, Gross Unrealized Losses	(574)	(162)
Total available for sale, Fair Value	216,950	271,391
Total available for sale, Carrying Value	216,950	271,391
Corporation
Schedule of Available-for-sale and Held-to-maturity Securities [Table] [Line Items]
Fixed maturity securities available for sale at cost	166,943	206,356
Cumulative effect adjustment resulting from changes in accounting principles	0	0
Total available for sale, Gross Unrealized Gains	7,191	8,755
Total available for sale, Gross Unrealized Losses	(1,532)	(638)
Total available for sale, Fair Value	172,602	214,473
Total available for sale, Carrying Value	172,602	214,473
Debt, Local Government
Schedule of Available-for-sale and Held-to-maturity Securities [Table] [Line Items]
Fixed maturity securities available for sale at cost	401,974	410,583
Cumulative effect adjustment resulting from changes in accounting principles	0	0
Total available for sale, Gross Unrealized Gains	29,455	40,596
Total available for sale, Gross Unrealized Losses	(732)	(555)
Total available for sale, Fair Value	430,697	450,624
Total available for sale, Carrying Value	430,697	450,624
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	125,709	181,105
Cumulative effect adjustment resulting from changes in accounting principles	0	0
Total available for sale, Gross Unrealized Gains	3,388	6,725
Total available for sale, Gross Unrealized Losses	(341)	(113)
Total available for sale, Fair Value	128,756	187,717
Total available for sale, Carrying Value	128,756	187,717
Mortgage-backed securities
Schedule of Available-for-sale and Held-to-maturity Securities [Table] [Line Items]
Fixed maturity securities available for sale at cost	1,066,453	994,292
Cumulative effect adjustment resulting from changes in accounting principles	0	0
Total available for sale, Gross Unrealized Gains	13,284	31,389
Total available for sale, Gross Unrealized Losses	(11,662)	(5,351)
Total available for sale, Fair Value	1,068,075	1,020,330
Total available for sale, Carrying Value	1,068,075	1,020,330
Asset-backed Securities
Schedule of Available-for-sale and Held-to-maturity Securities [Table] [Line Items]
Fixed maturity securities available for sale at cost	4,504,950	3,218,048
Cumulative effect adjustment resulting from changes in accounting principles	0	0
Total available for sale, Gross Unrealized Gains	4,409	10,035
Total available for sale, Gross Unrealized Losses	(18,794)	(33,497)
Total available for sale, Fair Value	4,490,565	3,194,586
Total available for sale, Carrying Value	4,490,565	3,194,586
Industrial Property
Schedule of Available-for-sale and Held-to-maturity Securities [Table] [Line Items]
Fixed maturity securities available for sale at cost	3,231,520	2,456,516
Cumulative effect adjustment resulting from changes in accounting principles	(16)	(518)
Total available for sale, Gross Unrealized Gains	62,751	115,926
Total available for sale, Gross Unrealized Losses	(21,092)	(7,449)
Total available for sale, Fair Value	3,273,163	2,564,475
Total available for sale, Carrying Value	3,273,163	2,564,475
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	1,739,282	1,513,943
Cumulative effect adjustment resulting from changes in accounting principles	0	0
Total available for sale, Gross Unrealized Gains	30,709	62,947
Total available for sale, Gross Unrealized Losses	(6,591)	(987)
Total available for sale, Fair Value	1,763,400	1,575,903
Total available for sale, Carrying Value	1,763,400	1,575,903
Public Utility, Bonds
Schedule of Available-for-sale and Held-to-maturity Securities [Table] [Line Items]
Fixed maturity securities available for sale at cost	396,242	389,267
Cumulative effect adjustment resulting from changes in accounting principles	0	0
Total available for sale, Gross Unrealized Gains	13,262	31,931
Total available for sale, Gross Unrealized Losses	(3,202)	(33)
Total available for sale, Fair Value	406,302	421,165
Total available for sale, Carrying Value	406,302	421,165
Other Security Investments
Schedule of Available-for-sale and Held-to-maturity Securities [Table] [Line Items]
Fixed maturity securities available for sale at cost	154,210	109,353
Cumulative effect adjustment resulting from changes in accounting principles	0	0
Total available for sale, Gross Unrealized Gains	125	696
Total available for sale, Gross Unrealized Losses	(1,525)	(11)
Total available for sale, Fair Value	152,810	110,038
Total available for sale, Carrying Value	152,810	110,038
Corporate
Schedule of Available-for-sale and Held-to-maturity Securities [Table] [Line Items]
Fixed maturity securities available for sale at cost	5,521,254	4,469,079
Cumulative effect adjustment resulting from changes in accounting principles	(16)	(518)	0
Total available for sale, Gross Unrealized Gains	106,847	211,500
Total available for sale, Gross Unrealized Losses	(32,410)	(8,480)
Total available for sale, Fair Value	5,595,675	4,671,581
Total available for sale, Carrying Value	5,595,675	4,671,581
Foreign government
Schedule of Available-for-sale and Held-to-maturity Securities [Table] [Line Items]
Fixed maturity securities available for sale at cost	1,277,109	993,268
Cumulative effect adjustment resulting from changes in accounting principles	(22,222)	(1,264)	$ 0
Total available for sale, Gross Unrealized Gains	7,508	28,007
Total available for sale, Gross Unrealized Losses	(47,494)	(44,448)
Total available for sale, Fair Value	1,214,901	975,563
Total available for sale, Carrying Value	$ 1,214,901	$ 975,563