Investments in Fixed Maturity Securities (Investments in fixed maturity securities) (Details) - USD ($) $ in Thousands	Mar. 31, 2023	Dec. 31, 2022	Mar. 31, 2022	Dec. 31, 2021
Debt Securities, Available-for-sale [Line Items]
Total held to maturity, Amortized Cost	$ 51,873	$ 51,410
Held-to-maturity, Allowance for Credit Loss	(107)	(114)	$ (378)	$ (387)
Held to maturity, Gross Unrealized Gains	3,955	4,277
Held to maturity, Gross Unrealized Losses	0	0
Total held to maturity, Fair Value	55,721	55,573
Total held to maturity, Carrying Value	51,766	51,296
Total available for sale, Amortized Cost	18,936,586	18,664,073
Available-for-sale, Allowance for Credit Loss	(37,142)	(37,352)	(26,153)	(22,238)
Total available for sale, Gross Unrealized Gains	56,556	32,166
Total available for sale, Gross Unrealized Losses	(916,653)	(1,122,834)
Total available for sale, Fair Value	18,039,347	17,536,053
Total available for sale, Carrying Value	18,039,347	17,536,053
Total investments in fixed maturity securities, Amortized Value	18,988,459	18,715,483
Fixed maturity securities, allowance for credit loss	(37,249)	(37,466)
Total investments in fixed maturity securities, Gross Unrealized Gains	60,511	36,443
Total investments in fixed maturity securities, Gross Unrealized Losses	(916,653)	(1,122,834)
Total investments in fixed maturity securities, Fair Value	18,095,068	17,591,626
Total investments in fixed maturity securities, Carrying Value	18,091,113	17,587,349
State and municipal
Debt Securities, Available-for-sale [Line Items]
Total held to maturity, Amortized Cost	48,474	47,802
Held-to-maturity, Allowance for Credit Loss	(107)	(114)
Held to maturity, Gross Unrealized Gains	3,902	4,239
Held to maturity, Gross Unrealized Losses	0	0
Total held to maturity, Fair Value	52,269	51,927
Total held to maturity, Carrying Value	48,367	47,688
Total available for sale, Amortized Cost	2,899,206	3,046,335
Available-for-sale, Allowance for Credit Loss	0	0
Total available for sale, Gross Unrealized Gains	17,573	12,355
Total available for sale, Gross Unrealized Losses	(127,420)	(168,592)
Total available for sale, Fair Value	2,789,359	2,890,098
Total available for sale, Carrying Value	2,789,359	2,890,098
Residential mortgage-backed
Debt Securities, Available-for-sale [Line Items]
Total held to maturity, Amortized Cost	3,399	3,608
Held-to-maturity, Allowance for Credit Loss	0	0
Held to maturity, Gross Unrealized Gains	53	38
Held to maturity, Gross Unrealized Losses	0	0
Total held to maturity, Fair Value	3,452	3,646
Total held to maturity, Carrying Value	3,399	3,608
Total available for sale, Amortized Cost	1,464,856	1,308,019
Available-for-sale, Allowance for Credit Loss	(23)	(18)
Total available for sale, Gross Unrealized Gains	2,298	395
Total available for sale, Gross Unrealized Losses	(153,147)	(171,595)
Total available for sale, Fair Value	1,313,984	1,136,801
Total available for sale, Carrying Value	1,313,984	1,136,801
U.S. government and government agency
Debt Securities, Available-for-sale [Line Items]
Total available for sale, Amortized Cost	1,100,204	960,479
Available-for-sale, Allowance for Credit Loss	0	0
Total available for sale, Gross Unrealized Gains	3,728	937
Total available for sale, Gross Unrealized Losses	(54,665)	(69,158)
Total available for sale, Fair Value	1,049,267	892,258
Total available for sale, Carrying Value	1,049,267	892,258
Special revenue
Debt Securities, Available-for-sale [Line Items]
Total available for sale, Amortized Cost	1,759,098	1,837,309
Available-for-sale, Allowance for Credit Loss	0	0
Total available for sale, Gross Unrealized Gains	5,429	3,662
Total available for sale, Gross Unrealized Losses	(90,044)	(119,474)
Total available for sale, Fair Value	1,674,483	1,721,497
Total available for sale, Carrying Value	1,674,483	1,721,497
State general obligation
Debt Securities, Available-for-sale [Line Items]
Total available for sale, Amortized Cost	386,952	387,709
Available-for-sale, Allowance for Credit Loss	0	0
Total available for sale, Gross Unrealized Gains	4,090	2,651
Total available for sale, Gross Unrealized Losses	(16,577)	(21,335)
Total available for sale, Fair Value	374,465	369,025
Total available for sale, Carrying Value	374,465	369,025
Pre-refunded
Debt Securities, Available-for-sale [Line Items]
Total available for sale, Amortized Cost	105,812	156,106
Available-for-sale, Allowance for Credit Loss	0	0
Total available for sale, Gross Unrealized Gains	3,351	2,741
Total available for sale, Gross Unrealized Losses	(248)	(7)
Total available for sale, Fair Value	108,915	158,840
Total available for sale, Carrying Value	108,915	158,840
Corporate backed
Debt Securities, Available-for-sale [Line Items]
Total available for sale, Amortized Cost	204,773	210,228
Available-for-sale, Allowance for Credit Loss	0	0
Total available for sale, Gross Unrealized Gains	491	334
Total available for sale, Gross Unrealized Losses	(8,447)	(10,923)
Total available for sale, Fair Value	196,817	199,639
Total available for sale, Carrying Value	196,817	199,639
Local general obligation
Debt Securities, Available-for-sale [Line Items]
Total available for sale, Amortized Cost	442,571	454,983
Available-for-sale, Allowance for Credit Loss	0	0
Total available for sale, Gross Unrealized Gains	4,212	2,967
Total available for sale, Gross Unrealized Losses	(12,104)	(16,853)
Total available for sale, Fair Value	434,679	441,097
Total available for sale, Carrying Value	434,679	441,097
Commercial
Debt Securities, Available-for-sale [Line Items]
Total available for sale, Amortized Cost	595,818	547,757
Available-for-sale, Allowance for Credit Loss	0	0
Total available for sale, Gross Unrealized Gains	1,220	215
Total available for sale, Gross Unrealized Losses	(13,293)	(19,363)
Total available for sale, Fair Value	583,745	528,609
Total available for sale, Carrying Value	583,745	528,609
Mortgage-backed
Debt Securities, Available-for-sale [Line Items]
Total available for sale, Amortized Cost	2,060,674	1,855,776
Available-for-sale, Allowance for Credit Loss	(23)	(18)
Total available for sale, Gross Unrealized Gains	3,518	610
Total available for sale, Gross Unrealized Losses	(166,440)	(190,958)
Total available for sale, Fair Value	1,897,729	1,665,410
Total available for sale, Carrying Value	1,897,729	1,665,428
Asset-backed
Debt Securities, Available-for-sale [Line Items]
Total available for sale, Amortized Cost	3,960,140	4,132,365
Available-for-sale, Allowance for Credit Loss	0	0
Total available for sale, Gross Unrealized Gains	3,166	2,730
Total available for sale, Gross Unrealized Losses	(117,853)	(152,322)
Total available for sale, Fair Value	3,845,453	3,982,773
Total available for sale, Carrying Value	3,845,453	3,982,773
Industrial
Debt Securities, Available-for-sale [Line Items]
Total available for sale, Amortized Cost	3,563,744	3,491,645
Available-for-sale, Allowance for Credit Loss	(66)	(1,704)
Total available for sale, Gross Unrealized Gains	9,599	4,439
Total available for sale, Gross Unrealized Losses	(187,833)	(241,381)
Total available for sale, Fair Value	3,385,444	3,252,999
Total available for sale, Carrying Value	3,385,444	3,252,999
Financial
Debt Securities, Available-for-sale [Line Items]
Total available for sale, Amortized Cost	2,737,880	2,585,247
Available-for-sale, Allowance for Credit Loss	(3,729)	(2,997)
Total available for sale, Gross Unrealized Gains	7,421	5,505
Total available for sale, Gross Unrealized Losses	(115,268)	(117,383)
Total available for sale, Fair Value	2,626,304	2,470,372
Total available for sale, Carrying Value	2,626,304	2,470,372
Utilities
Debt Securities, Available-for-sale [Line Items]
Total available for sale, Amortized Cost	650,151	586,066
Available-for-sale, Allowance for Credit Loss	0	0
Total available for sale, Gross Unrealized Gains	5,117	1,307
Total available for sale, Gross Unrealized Losses	(27,941)	(36,325)
Total available for sale, Fair Value	627,327	551,048
Total available for sale, Carrying Value	627,327	551,048
Other
Debt Securities, Available-for-sale [Line Items]
Total available for sale, Amortized Cost	495,850	441,230
Available-for-sale, Allowance for Credit Loss	0	0
Total available for sale, Gross Unrealized Gains	122	0
Total available for sale, Gross Unrealized Losses	(9,398)	(11,657)
Total available for sale, Fair Value	486,574	429,573
Total available for sale, Carrying Value	486,574	429,573
Corporate
Debt Securities, Available-for-sale [Line Items]
Total available for sale, Amortized Cost	7,447,625	7,104,188
Available-for-sale, Allowance for Credit Loss	(3,795)	(4,701)	0	(16)
Total available for sale, Gross Unrealized Gains	22,259	11,251
Total available for sale, Gross Unrealized Losses	(340,440)	(406,746)
Total available for sale, Fair Value	7,125,649	6,703,992
Total available for sale, Carrying Value	7,125,649	6,703,992
Foreign government
Debt Securities, Available-for-sale [Line Items]
Total available for sale, Amortized Cost	1,468,737	1,564,930
Available-for-sale, Allowance for Credit Loss	(33,324)	(32,633)	$ (26,153)	$ (22,222)
Total available for sale, Gross Unrealized Gains	6,312	4,283
Total available for sale, Gross Unrealized Losses	(109,835)	(135,058)
Total available for sale, Fair Value	1,331,890	1,401,522
Total available for sale, Carrying Value	$ 1,331,890	$ 1,401,522