Investments in Fixed Maturity Securities (Investments in fixed maturity securities) (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Jun. 30, 2021	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Dec. 31, 2019
Debt Securities, Available-for-sale [Line Items]
Total held to maturity, Amortized Cost	$ 74,170		$ 73,572
Held-to-maturity, Allowance for Credit Loss	(396)	$ (453)	(798)	$ (871)	$ (948)	$ 0
Held to maturity, Gross Unrealized Gains	11,322		14,260
Held to maturity, Gross Unrealized Losses	0		0
Total held to maturity, Fair Value	85,096		87,034
Total held to maturity, Carrying Value	73,774		72,774
Total available for sale, Amortized Cost	15,734,858		13,682,286
Available-for-sale, Allowance for Credit Loss	(16,887)	(18,911)	(1,782)	(1,696)	(45,493)	0
Total available for sale, Gross Unrealized Gains	358,001		500,283
Total available for sale, Gross Unrealized Losses	(76,611)		(94,192)
Total available for sale, Fair Value	15,999,361		14,086,595
Total available for sale, Carrying Value	15,999,361		14,086,595
Total investments in fixed maturity securities, Amortized Value	15,809,028		13,755,858
Fixed maturity securities, allowance for credit loss	(17,283)		(2,580)
Total investments in fixed maturity securities, Gross Unrealized Gains	369,323		514,543
Total investments in fixed maturity securities, Gross Unrealized Losses	(76,611)		(94,192)
Total investments in fixed maturity securities, Fair Value	16,084,457		14,173,629
Total investments in fixed maturity securities, Carrying Value	16,073,135		14,159,369
State and municipal
Debt Securities, Available-for-sale [Line Items]
Total held to maturity, Amortized Cost	68,920		67,117
Held-to-maturity, Allowance for Credit Loss	(396)		(798)
Held to maturity, Gross Unrealized Gains	10,548		13,217
Held to maturity, Gross Unrealized Losses	0		0
Total held to maturity, Fair Value	79,072		79,536
Total held to maturity, Carrying Value	68,524		66,319
Total available for sale, Amortized Cost	3,177,560		3,421,579
Available-for-sale, Allowance for Credit Loss	0		0
Total available for sale, Gross Unrealized Gains	158,120		201,154
Total available for sale, Gross Unrealized Losses	(6,335)		(2,069)
Total available for sale, Fair Value	3,329,345		3,620,664
Total available for sale, Carrying Value	3,329,345		3,620,664
Residential mortgage-backed
Debt Securities, Available-for-sale [Line Items]
Total held to maturity, Amortized Cost	5,250		6,455
Held-to-maturity, Allowance for Credit Loss	0		0
Held to maturity, Gross Unrealized Gains	774		1,043
Held to maturity, Gross Unrealized Losses	0		0
Total held to maturity, Fair Value	6,024		7,498
Total held to maturity, Carrying Value	5,250		6,455
Total available for sale, Amortized Cost	835,729		813,187
Available-for-sale, Allowance for Credit Loss	0		0
Total available for sale, Gross Unrealized Gains	14,156		24,664
Total available for sale, Gross Unrealized Losses	(7,185)		(5,238)
Total available for sale, Fair Value	842,700		832,613
Total available for sale, Carrying Value	842,700		832,613
U.S. government and government agency
Debt Securities, Available-for-sale [Line Items]
Total available for sale, Amortized Cost	506,703		586,020
Available-for-sale, Allowance for Credit Loss	0		0
Total available for sale, Gross Unrealized Gains	12,137		18,198
Total available for sale, Gross Unrealized Losses	(507)		(347)
Total available for sale, Fair Value	518,333		603,871
Total available for sale, Carrying Value	518,333		603,871
Special revenue
Debt Securities, Available-for-sale [Line Items]
Total available for sale, Amortized Cost	2,019,757		2,137,162
Available-for-sale, Allowance for Credit Loss	0		0
Total available for sale, Gross Unrealized Gains	74,682		96,924
Total available for sale, Gross Unrealized Losses	(2,967)		(714)
Total available for sale, Fair Value	2,091,472		2,233,372
Total available for sale, Carrying Value	2,091,472		2,233,372
State general obligation
Debt Securities, Available-for-sale [Line Items]
Total available for sale, Amortized Cost	376,534		417,397
Available-for-sale, Allowance for Credit Loss	0		0
Total available for sale, Gross Unrealized Gains	26,967		33,407
Total available for sale, Gross Unrealized Losses	(629)		0
Total available for sale, Fair Value	402,872		450,804
Total available for sale, Carrying Value	402,872		450,804
Pre-refunded
Debt Securities, Available-for-sale [Line Items]
Total available for sale, Amortized Cost	209,547		250,081
Available-for-sale, Allowance for Credit Loss	0		0
Total available for sale, Gross Unrealized Gains	16,933		21,472
Total available for sale, Gross Unrealized Losses	(917)		(162)
Total available for sale, Fair Value	225,563		271,391
Total available for sale, Carrying Value	225,563		271,391
Corporate backed
Debt Securities, Available-for-sale [Line Items]
Total available for sale, Amortized Cost	170,962		206,356
Available-for-sale, Allowance for Credit Loss	0		0
Total available for sale, Gross Unrealized Gains	8,271		8,755
Total available for sale, Gross Unrealized Losses	(1,317)		(638)
Total available for sale, Fair Value	177,916		214,473
Total available for sale, Carrying Value	177,916		214,473
Local general obligation
Debt Securities, Available-for-sale [Line Items]
Total available for sale, Amortized Cost	400,760		410,583
Available-for-sale, Allowance for Credit Loss	0		0
Total available for sale, Gross Unrealized Gains	31,267		40,596
Total available for sale, Gross Unrealized Losses	(505)		(555)
Total available for sale, Fair Value	431,522		450,624
Total available for sale, Carrying Value	431,522		450,624
Commercial
Debt Securities, Available-for-sale [Line Items]
Total available for sale, Amortized Cost	125,768		181,105
Available-for-sale, Allowance for Credit Loss	0		0
Total available for sale, Gross Unrealized Gains	4,979		6,725
Total available for sale, Gross Unrealized Losses	(110)		(113)
Total available for sale, Fair Value	130,637		187,717
Total available for sale, Carrying Value	130,637		187,717
Mortgage-backed
Debt Securities, Available-for-sale [Line Items]
Total available for sale, Amortized Cost	961,497		994,292
Available-for-sale, Allowance for Credit Loss	0		0
Total available for sale, Gross Unrealized Gains	19,135		31,389
Total available for sale, Gross Unrealized Losses	(7,295)		(5,351)
Total available for sale, Fair Value	973,337		1,020,330
Total available for sale, Carrying Value	973,337		1,020,330
Asset-backed
Debt Securities, Available-for-sale [Line Items]
Total available for sale, Amortized Cost	4,660,976		3,218,048
Available-for-sale, Allowance for Credit Loss	0		0
Total available for sale, Gross Unrealized Gains	8,780		10,035
Total available for sale, Gross Unrealized Losses	(14,201)		(33,497)
Total available for sale, Fair Value	4,655,555		3,194,586
Total available for sale, Carrying Value	4,655,555		3,194,586
Industrial
Debt Securities, Available-for-sale [Line Items]
Total available for sale, Amortized Cost	3,057,258		2,456,516
Available-for-sale, Allowance for Credit Loss	(12)		(518)
Total available for sale, Gross Unrealized Gains	85,049		115,926
Total available for sale, Gross Unrealized Losses	(9,933)		(7,449)
Total available for sale, Fair Value	3,132,362		2,564,475
Total available for sale, Carrying Value	3,132,362		2,564,475
Financial
Debt Securities, Available-for-sale [Line Items]
Total available for sale, Amortized Cost	1,657,059		1,513,943
Available-for-sale, Allowance for Credit Loss	0		0
Total available for sale, Gross Unrealized Gains	44,534		62,947
Total available for sale, Gross Unrealized Losses	(1,753)		(987)
Total available for sale, Fair Value	1,699,840		1,575,903
Total available for sale, Carrying Value	1,699,840		1,575,903
Utilities
Debt Securities, Available-for-sale [Line Items]
Total available for sale, Amortized Cost	402,952		389,267
Available-for-sale, Allowance for Credit Loss	0		0
Total available for sale, Gross Unrealized Gains	17,228		31,931
Total available for sale, Gross Unrealized Losses	(1,327)		(33)
Total available for sale, Fair Value	418,853		421,165
Total available for sale, Carrying Value	418,853		421,165
Other
Debt Securities, Available-for-sale [Line Items]
Total available for sale, Amortized Cost	173,504		109,353
Available-for-sale, Allowance for Credit Loss	0		0
Total available for sale, Gross Unrealized Gains	154		696
Total available for sale, Gross Unrealized Losses	(649)		(11)
Total available for sale, Fair Value	173,009		110,038
Total available for sale, Carrying Value	173,009		110,038
Corporate
Debt Securities, Available-for-sale [Line Items]
Total available for sale, Amortized Cost	5,290,773		4,469,079
Available-for-sale, Allowance for Credit Loss	(12)	(12)	(518)	(606)	(724)	0
Total available for sale, Gross Unrealized Gains	146,965		211,500
Total available for sale, Gross Unrealized Losses	(13,662)		(8,480)
Total available for sale, Fair Value	5,424,064		4,671,581
Total available for sale, Carrying Value	5,424,064		4,671,581
Foreign government
Debt Securities, Available-for-sale [Line Items]
Total available for sale, Amortized Cost	1,137,349		993,268
Available-for-sale, Allowance for Credit Loss	(16,875)	$ (18,899)	(1,264)	$ (1,090)	$ (44,769)	$ 0
Total available for sale, Gross Unrealized Gains	12,864		28,007
Total available for sale, Gross Unrealized Losses	(34,611)		(44,448)
Total available for sale, Fair Value	1,098,727		975,563
Total available for sale, Carrying Value	$ 1,098,727		$ 975,563