Investments in Fixed Maturity Securities (Investments in fixed maturity securities) (Details) - USD ($) $ in Thousands	Mar. 31, 2022	Dec. 31, 2021	Mar. 31, 2021	Dec. 31, 2020
Debt Securities, Available-for-sale [Line Items]
Total held to maturity, Amortized Cost	$ 74,620	$ 74,368
Held-to-maturity, Allowance for Credit Loss	(378)	(387)	$ (730)	$ (798)
Held to maturity, Gross Unrealized Gains	7,418	11,445
Held to maturity, Gross Unrealized Losses	0	0
Total held to maturity, Fair Value	81,660	85,426
Total held to maturity, Carrying Value	74,242	73,981
Total available for sale, Amortized Cost	16,764,083	16,396,936
Available-for-sale, Allowance for Credit Loss	(26,153)	(22,238)	(20,009)	(1,782)
Total available for sale, Gross Unrealized Gains	67,990	278,207
Total available for sale, Gross Unrealized Losses	(453,966)	(124,213)
Total available for sale, Fair Value	16,351,954	16,528,692
Total available for sale, Carrying Value	16,351,954	16,528,692
Total investments in fixed maturity securities, Amortized Value	16,838,703	16,471,304
Fixed maturity securities, allowance for credit loss	(26,531)	(22,625)
Total investments in fixed maturity securities, Gross Unrealized Gains	75,408	289,652
Total investments in fixed maturity securities, Gross Unrealized Losses	(453,966)	(124,213)
Total investments in fixed maturity securities, Fair Value	16,433,614	16,614,118
Total investments in fixed maturity securities, Carrying Value	16,426,196	16,602,673
State and municipal
Debt Securities, Available-for-sale [Line Items]
Total held to maturity, Amortized Cost	70,165	69,539
Held-to-maturity, Allowance for Credit Loss	(378)	(387)
Held to maturity, Gross Unrealized Gains	7,094	10,813
Held to maturity, Gross Unrealized Losses	0	0
Total held to maturity, Fair Value	76,881	79,965
Total held to maturity, Carrying Value	69,787	69,152
Total available for sale, Amortized Cost	3,078,004	3,176,042
Available-for-sale, Allowance for Credit Loss	0	0
Total available for sale, Gross Unrealized Gains	38,794	137,650
Total available for sale, Gross Unrealized Losses	(60,924)	(9,559)
Total available for sale, Fair Value	3,055,874	3,304,133
Total available for sale, Carrying Value	3,055,874	3,304,133
Residential mortgage-backed
Debt Securities, Available-for-sale [Line Items]
Total held to maturity, Amortized Cost	4,455	4,829
Held-to-maturity, Allowance for Credit Loss	0	0
Held to maturity, Gross Unrealized Gains	324	632
Held to maturity, Gross Unrealized Losses	0	0
Total held to maturity, Fair Value	4,779	5,461
Total held to maturity, Carrying Value	4,455	4,829
Total available for sale, Amortized Cost	1,179,468	940,744
Available-for-sale, Allowance for Credit Loss	0	0
Total available for sale, Gross Unrealized Gains	2,417	9,896
Total available for sale, Gross Unrealized Losses	(60,650)	(11,321)
Total available for sale, Fair Value	1,121,235	939,319
Total available for sale, Carrying Value	1,121,235	939,319
U.S. government and government agency
Debt Securities, Available-for-sale [Line Items]
Total available for sale, Amortized Cost	837,167	851,128
Available-for-sale, Allowance for Credit Loss	0	0
Total available for sale, Gross Unrealized Gains	2,397	8,509
Total available for sale, Gross Unrealized Losses	(20,716)	(4,294)
Total available for sale, Fair Value	818,848	855,343
Total available for sale, Carrying Value	818,848	855,343
Special revenue
Debt Securities, Available-for-sale [Line Items]
Total available for sale, Amortized Cost	1,948,586	2,016,382
Available-for-sale, Allowance for Credit Loss	0	0
Total available for sale, Gross Unrealized Gains	12,944	62,961
Total available for sale, Gross Unrealized Losses	(44,803)	(5,706)
Total available for sale, Fair Value	1,916,727	2,073,637
Total available for sale, Carrying Value	1,916,727	2,073,637
State general obligation
Debt Securities, Available-for-sale [Line Items]
Total available for sale, Amortized Cost	378,408	388,110
Available-for-sale, Allowance for Credit Loss	0	0
Total available for sale, Gross Unrealized Gains	7,043	23,152
Total available for sale, Gross Unrealized Losses	(6,574)	(1,015)
Total available for sale, Fair Value	378,877	410,247
Total available for sale, Carrying Value	378,877	410,247
Pre-refunded
Debt Securities, Available-for-sale [Line Items]
Total available for sale, Amortized Cost	165,882	202,633
Available-for-sale, Allowance for Credit Loss	0	0
Total available for sale, Gross Unrealized Gains	6,216	14,891
Total available for sale, Gross Unrealized Losses	0	(574)
Total available for sale, Fair Value	172,098	216,950
Total available for sale, Carrying Value	172,098	216,950
Corporate backed
Debt Securities, Available-for-sale [Line Items]
Total available for sale, Amortized Cost	175,070	166,943
Available-for-sale, Allowance for Credit Loss	0	0
Total available for sale, Gross Unrealized Gains	1,407	7,191
Total available for sale, Gross Unrealized Losses	(4,472)	(1,532)
Total available for sale, Fair Value	172,005	172,602
Total available for sale, Carrying Value	172,005	172,602
Local general obligation
Debt Securities, Available-for-sale [Line Items]
Total available for sale, Amortized Cost	410,058	401,974
Available-for-sale, Allowance for Credit Loss	0	0
Total available for sale, Gross Unrealized Gains	11,184	29,455
Total available for sale, Gross Unrealized Losses	(5,075)	(732)
Total available for sale, Fair Value	416,167	430,697
Total available for sale, Carrying Value	416,167	430,697
Commercial
Debt Securities, Available-for-sale [Line Items]
Total available for sale, Amortized Cost	266,921	125,709
Available-for-sale, Allowance for Credit Loss	0	0
Total available for sale, Gross Unrealized Gains	339	3,388
Total available for sale, Gross Unrealized Losses	(3,759)	(341)
Total available for sale, Fair Value	263,501	128,756
Total available for sale, Carrying Value	263,501	128,756
Mortgage-backed
Debt Securities, Available-for-sale [Line Items]
Total available for sale, Amortized Cost	1,446,389	1,066,453
Available-for-sale, Allowance for Credit Loss	0	0
Total available for sale, Gross Unrealized Gains	2,756	13,284
Total available for sale, Gross Unrealized Losses	(64,409)	(11,662)
Total available for sale, Fair Value	1,384,736	1,068,075
Total available for sale, Carrying Value	1,384,736	1,068,075
Asset-backed
Debt Securities, Available-for-sale [Line Items]
Total available for sale, Amortized Cost	4,319,535	4,504,950
Available-for-sale, Allowance for Credit Loss	0	0
Total available for sale, Gross Unrealized Gains	1,459	4,409
Total available for sale, Gross Unrealized Losses	(63,573)	(18,794)
Total available for sale, Fair Value	4,257,421	4,490,565
Total available for sale, Carrying Value	4,257,421	4,490,565
Industrial
Debt Securities, Available-for-sale [Line Items]
Total available for sale, Amortized Cost	3,373,710	3,231,520
Available-for-sale, Allowance for Credit Loss		(16)
Total available for sale, Gross Unrealized Gains	12,708	62,751
Total available for sale, Gross Unrealized Losses	(115,614)	(21,092)
Total available for sale, Fair Value	3,270,804	3,273,163
Total available for sale, Carrying Value	3,270,804	3,273,163
Financial
Debt Securities, Available-for-sale [Line Items]
Total available for sale, Amortized Cost	1,747,275	1,739,282
Available-for-sale, Allowance for Credit Loss	0	0
Total available for sale, Gross Unrealized Gains	4,612	30,709
Total available for sale, Gross Unrealized Losses	(45,155)	(6,591)
Total available for sale, Fair Value	1,706,732	1,763,400
Total available for sale, Carrying Value	1,706,732	1,763,400
Utilities
Debt Securities, Available-for-sale [Line Items]
Total available for sale, Amortized Cost	422,354	396,242
Available-for-sale, Allowance for Credit Loss	0	0
Total available for sale, Gross Unrealized Gains	1,806	13,262
Total available for sale, Gross Unrealized Losses	(14,434)	(3,202)
Total available for sale, Fair Value	409,726	406,302
Total available for sale, Carrying Value	409,726	406,302
Other
Debt Securities, Available-for-sale [Line Items]
Total available for sale, Amortized Cost	206,526	154,210
Available-for-sale, Allowance for Credit Loss	0	0
Total available for sale, Gross Unrealized Gains	47	125
Total available for sale, Gross Unrealized Losses	(5,328)	(1,525)
Total available for sale, Fair Value	201,245	152,810
Total available for sale, Carrying Value	201,245	152,810
Corporate
Debt Securities, Available-for-sale [Line Items]
Total available for sale, Amortized Cost	5,749,865	5,521,254
Available-for-sale, Allowance for Credit Loss	0	(16)	(16)	(518)
Total available for sale, Gross Unrealized Gains	19,173	106,847
Total available for sale, Gross Unrealized Losses	(180,531)	(32,410)
Total available for sale, Fair Value	5,588,507	5,595,675
Total available for sale, Carrying Value	5,588,507	5,595,675
Foreign government
Debt Securities, Available-for-sale [Line Items]
Total available for sale, Amortized Cost	1,333,123	1,277,109
Available-for-sale, Allowance for Credit Loss	(26,153)	(22,222)	$ (19,993)	$ (1,264)
Total available for sale, Gross Unrealized Gains	3,411	7,508
Total available for sale, Gross Unrealized Losses	(63,813)	(47,494)
Total available for sale, Fair Value	1,246,568	1,214,901
Total available for sale, Carrying Value	$ 1,246,568	$ 1,214,901