Investments in Fixed Maturity Securities (Investments in fixed maturity securities) (Details) - USD ($) $ in Thousands	Jun. 30, 2021	Mar. 31, 2021	Dec. 31, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019
Debt Securities, Available-for-sale [Line Items]
Total held to maturity, Amortized Cost	$ 74,034		$ 73,572
Held-to-maturity, Allowance for Credit Loss	(453)	$ (730)	(798)	$ (948)	$ (107)	$ 0
Held to maturity, Gross Unrealized Gains	12,928		14,260
Held to maturity, Gross Unrealized Losses	0		0
Total held to maturity, Fair Value	86,509		87,034
Total held to maturity, Carrying Value	73,581		72,774
Total available for sale, Amortized Cost	15,276,519		13,682,286
Available-for-sale, Allowance for Credit Loss	(18,911)	(20,009)	(1,782)	(45,493)	(67,356)	0
Total available for sale, Gross Unrealized Gains	401,998		500,283
Total available for sale, Gross Unrealized Losses	(79,679)		(94,192)
Total available for sale, Fair Value	15,579,927		14,086,595
Total available for sale, Carrying Value	15,579,927		14,086,595
Total investments in fixed maturity securities, Amortized Value	15,350,553		13,755,858
Fixed maturity securities, allowance for credit loss	(19,364)		(2,580)
Total investments in fixed maturity securities, Gross Unrealized Gains	414,926		514,543
Total investments in fixed maturity securities, Gross Unrealized Losses	(79,679)		(94,192)
Total investments in fixed maturity securities, Fair Value	15,666,436		14,173,629
Total investments in fixed maturity securities, Carrying Value	15,653,508		14,159,369
State and municipal
Debt Securities, Available-for-sale [Line Items]
Total held to maturity, Amortized Cost	68,310		67,117
Held-to-maturity, Allowance for Credit Loss	(453)		(798)
Held to maturity, Gross Unrealized Gains	12,069		13,217
Held to maturity, Gross Unrealized Losses	0		0
Total held to maturity, Fair Value	79,926		79,536
Total held to maturity, Carrying Value	67,857		66,319
Total available for sale, Amortized Cost	3,254,895		3,421,579
Available-for-sale, Allowance for Credit Loss	0		0
Total available for sale, Gross Unrealized Gains	174,839		201,154
Total available for sale, Gross Unrealized Losses	(3,578)		(2,069)
Total available for sale, Fair Value	3,426,156		3,620,664
Total available for sale, Carrying Value	3,426,156		3,620,664
Residential mortgage-backed
Debt Securities, Available-for-sale [Line Items]
Total held to maturity, Amortized Cost	5,724		6,455
Held-to-maturity, Allowance for Credit Loss	0		0
Held to maturity, Gross Unrealized Gains	859		1,043
Held to maturity, Gross Unrealized Losses	0		0
Total held to maturity, Fair Value	6,583		7,498
Total held to maturity, Carrying Value	5,724		6,455
Total available for sale, Amortized Cost	771,507		813,187
Available-for-sale, Allowance for Credit Loss	0		0
Total available for sale, Gross Unrealized Gains	16,798		24,664
Total available for sale, Gross Unrealized Losses	(8,189)		(5,238)
Total available for sale, Fair Value	780,116		832,613
Total available for sale, Carrying Value	780,116		832,613
U.S. government and government agency
Debt Securities, Available-for-sale [Line Items]
Total available for sale, Amortized Cost	488,196		586,020
Available-for-sale, Allowance for Credit Loss	0		0
Total available for sale, Gross Unrealized Gains	14,890		18,198
Total available for sale, Gross Unrealized Losses	(461)		(347)
Total available for sale, Fair Value	502,625		603,871
Total available for sale, Carrying Value	502,625		603,871
Special revenue
Debt Securities, Available-for-sale [Line Items]
Total available for sale, Amortized Cost	2,074,036		2,137,162
Available-for-sale, Allowance for Credit Loss	0		0
Total available for sale, Gross Unrealized Gains	85,037		96,924
Total available for sale, Gross Unrealized Losses	(1,433)		(714)
Total available for sale, Fair Value	2,157,640		2,233,372
Total available for sale, Carrying Value	2,157,640		2,233,372
State general obligation
Debt Securities, Available-for-sale [Line Items]
Total available for sale, Amortized Cost	378,596		417,397
Available-for-sale, Allowance for Credit Loss	0		0
Total available for sale, Gross Unrealized Gains	29,413		33,407
Total available for sale, Gross Unrealized Losses	(250)		0
Total available for sale, Fair Value	407,759		450,804
Total available for sale, Carrying Value	407,759		450,804
Pre-refunded
Debt Securities, Available-for-sale [Line Items]
Total available for sale, Amortized Cost	225,787		250,081
Available-for-sale, Allowance for Credit Loss	0		0
Total available for sale, Gross Unrealized Gains	17,677		21,472
Total available for sale, Gross Unrealized Losses	(607)		(162)
Total available for sale, Fair Value	242,857		271,391
Total available for sale, Carrying Value	242,857		271,391
Corporate backed
Debt Securities, Available-for-sale [Line Items]
Total available for sale, Amortized Cost	156,970		206,356
Available-for-sale, Allowance for Credit Loss	0		0
Total available for sale, Gross Unrealized Gains	9,082		8,755
Total available for sale, Gross Unrealized Losses	(862)		(638)
Total available for sale, Fair Value	165,190		214,473
Total available for sale, Carrying Value	165,190		214,473
Local general obligation
Debt Securities, Available-for-sale [Line Items]
Total available for sale, Amortized Cost	419,506		410,583
Available-for-sale, Allowance for Credit Loss	0		0
Total available for sale, Gross Unrealized Gains	33,630		40,596
Total available for sale, Gross Unrealized Losses	(426)		(555)
Total available for sale, Fair Value	452,710		450,624
Total available for sale, Carrying Value	452,710		450,624
Commercial
Debt Securities, Available-for-sale [Line Items]
Total available for sale, Amortized Cost	137,996		181,105
Available-for-sale, Allowance for Credit Loss	0		0
Total available for sale, Gross Unrealized Gains	5,740		6,725
Total available for sale, Gross Unrealized Losses	(150)		(113)
Total available for sale, Fair Value	143,586		187,717
Total available for sale, Carrying Value	143,586		187,717
Mortgage-backed
Debt Securities, Available-for-sale [Line Items]
Total available for sale, Amortized Cost	909,503		994,292
Available-for-sale, Allowance for Credit Loss	0		0
Total available for sale, Gross Unrealized Gains	22,538		31,389
Total available for sale, Gross Unrealized Losses	(8,339)		(5,351)
Total available for sale, Fair Value	923,702		1,020,330
Total available for sale, Carrying Value	923,702		1,020,330
Asset-backed
Debt Securities, Available-for-sale [Line Items]
Total available for sale, Amortized Cost	4,333,543		3,218,048
Available-for-sale, Allowance for Credit Loss	0		0
Total available for sale, Gross Unrealized Gains	8,449		10,035
Total available for sale, Gross Unrealized Losses	(24,129)		(33,497)
Total available for sale, Fair Value	4,317,863		3,194,586
Total available for sale, Carrying Value	4,317,863		3,194,586
Industrial
Debt Securities, Available-for-sale [Line Items]
Total available for sale, Amortized Cost	3,019,720		2,456,516
Available-for-sale, Allowance for Credit Loss	(12)		(518)
Total available for sale, Gross Unrealized Gains	94,056		115,926
Total available for sale, Gross Unrealized Losses	(7,761)		(7,449)
Total available for sale, Fair Value	3,106,003		2,564,475
Total available for sale, Carrying Value	3,106,003		2,564,475
Financial
Debt Securities, Available-for-sale [Line Items]
Total available for sale, Amortized Cost	1,583,291		1,513,943
Available-for-sale, Allowance for Credit Loss	0		0
Total available for sale, Gross Unrealized Gains	51,103		62,947
Total available for sale, Gross Unrealized Losses	(1,076)		(987)
Total available for sale, Fair Value	1,633,318		1,575,903
Total available for sale, Carrying Value	1,633,318		1,575,903
Utilities
Debt Securities, Available-for-sale [Line Items]
Total available for sale, Amortized Cost	406,423		389,267
Available-for-sale, Allowance for Credit Loss	0		0
Total available for sale, Gross Unrealized Gains	18,531		31,931
Total available for sale, Gross Unrealized Losses	(1,106)		(33)
Total available for sale, Fair Value	423,848		421,165
Total available for sale, Carrying Value	423,848		421,165
Other
Debt Securities, Available-for-sale [Line Items]
Total available for sale, Amortized Cost	177,445		109,353
Available-for-sale, Allowance for Credit Loss	0		0
Total available for sale, Gross Unrealized Gains	416		696
Total available for sale, Gross Unrealized Losses	(367)		(11)
Total available for sale, Fair Value	177,494		110,038
Total available for sale, Carrying Value	177,494		110,038
Corporate
Debt Securities, Available-for-sale [Line Items]
Total available for sale, Amortized Cost	5,186,879		4,469,079
Available-for-sale, Allowance for Credit Loss	(12)	(16)	(518)	(724)	(6,436)	0
Total available for sale, Gross Unrealized Gains	164,106		211,500
Total available for sale, Gross Unrealized Losses	(10,310)		(8,480)
Total available for sale, Fair Value	5,340,663		4,671,581
Total available for sale, Carrying Value	5,340,663		4,671,581
Foreign government
Debt Securities, Available-for-sale [Line Items]
Total available for sale, Amortized Cost	1,103,503		993,268
Available-for-sale, Allowance for Credit Loss	(18,899)	$ (19,993)	(1,264)	$ (44,769)	$ (60,920)	$ 0
Total available for sale, Gross Unrealized Gains	17,176		28,007
Total available for sale, Gross Unrealized Losses	(32,862)		(44,448)
Total available for sale, Fair Value	1,068,918		975,563
Total available for sale, Carrying Value	$ 1,068,918		$ 975,563