Investments in Fixed Maturity Securities (Investments in fixed maturity securities) (Details) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020	Mar. 31, 2020	Dec. 31, 2019
Debt Securities, Available-for-sale [Line Items]
Total held to maturity, Amortized Cost	$ 73,811	$ 73,572
Held-to-maturity, Allowance for Credit Loss	(730)	(798)	$ (107)	$ 0
Held to maturity, Gross Unrealized Gains	12,074	14,260
Held to maturity, Gross Unrealized Losses	0	0
Total held to maturity, Fair Value	85,155	87,034
Total held to maturity, Carrying Value	73,081	72,774
Total available for sale, Amortized Cost	15,053,169	13,682,286
Available-for-sale, Allowance for Credit Loss	(20,009)	(1,782)	(67,356)	0
Total available for sale, Gross Unrealized Gains	392,971	500,283
Total available for sale, Gross Unrealized Losses	(99,094)	(94,192)
Total available for sale, Fair Value	15,327,037	14,086,595
Total available for sale, Carrying Value	15,327,037	14,086,595
Total investments in fixed maturity securities, Amortized Value	15,126,980	13,755,858
Fixed maturity securities, allowance for credit loss	(20,739)	(2,580)
Total investments in fixed maturity securities, Gross Unrealized Gains	405,045	514,543
Total investments in fixed maturity securities, Gross Unrealized Losses	(99,094)	(94,192)
Total investments in fixed maturity securities, Fair Value	15,412,192	14,173,629
Total investments in fixed maturity securities, Carrying Value	15,400,118	14,159,369
State and municipal
Debt Securities, Available-for-sale [Line Items]
Total held to maturity, Amortized Cost	67,710	67,117
Held-to-maturity, Allowance for Credit Loss	(730)	(798)
Held to maturity, Gross Unrealized Gains	11,150	13,217
Held to maturity, Gross Unrealized Losses	0	0
Total held to maturity, Fair Value	78,130	79,536
Total held to maturity, Carrying Value	66,980	66,319
Total available for sale, Amortized Cost	3,340,743	3,421,579
Available-for-sale, Allowance for Credit Loss	0	0
Total available for sale, Gross Unrealized Gains	168,031	201,154
Total available for sale, Gross Unrealized Losses	(6,181)	(2,069)
Total available for sale, Fair Value	3,502,593	3,620,664
Total available for sale, Carrying Value	3,502,593	3,620,664
Residential mortgage-backed
Debt Securities, Available-for-sale [Line Items]
Total held to maturity, Amortized Cost	6,101	6,455
Held-to-maturity, Allowance for Credit Loss	0	0
Held to maturity, Gross Unrealized Gains	924	1,043
Held to maturity, Gross Unrealized Losses	0	0
Total held to maturity, Fair Value	7,025	7,498
Total held to maturity, Carrying Value	6,101	6,455
Total available for sale, Amortized Cost	889,638	813,187
Available-for-sale, Allowance for Credit Loss	0	0
Total available for sale, Gross Unrealized Gains	19,315	24,664
Total available for sale, Gross Unrealized Losses	(12,215)	(5,238)
Total available for sale, Fair Value	896,738	832,613
Total available for sale, Carrying Value	896,738	832,613
U.S. government and government agency
Debt Securities, Available-for-sale [Line Items]
Total available for sale, Amortized Cost	541,180	586,020
Available-for-sale, Allowance for Credit Loss	0	0
Total available for sale, Gross Unrealized Gains	15,449	18,198
Total available for sale, Gross Unrealized Losses	(915)	(347)
Total available for sale, Fair Value	555,714	603,871
Total available for sale, Carrying Value	555,714	603,871
Special revenue
Debt Securities, Available-for-sale [Line Items]
Total available for sale, Amortized Cost	2,112,542	2,137,162
Available-for-sale, Allowance for Credit Loss	0	0
Total available for sale, Gross Unrealized Gains	78,450	96,924
Total available for sale, Gross Unrealized Losses	(3,284)	(714)
Total available for sale, Fair Value	2,187,708	2,233,372
Total available for sale, Carrying Value	2,187,708	2,233,372
State general obligation
Debt Securities, Available-for-sale [Line Items]
Total available for sale, Amortized Cost	399,636	417,397
Available-for-sale, Allowance for Credit Loss	0	0
Total available for sale, Gross Unrealized Gains	28,156	33,407
Total available for sale, Gross Unrealized Losses	(1,059)	0
Total available for sale, Fair Value	426,733	450,804
Total available for sale, Carrying Value	426,733	450,804
Pre-refunded
Debt Securities, Available-for-sale [Line Items]
Total available for sale, Amortized Cost	231,235	250,081
Available-for-sale, Allowance for Credit Loss	0	0
Total available for sale, Gross Unrealized Gains	17,990	21,472
Total available for sale, Gross Unrealized Losses	(465)	(162)
Total available for sale, Fair Value	248,760	271,391
Total available for sale, Carrying Value	248,760	271,391
Corporate backed
Debt Securities, Available-for-sale [Line Items]
Total available for sale, Amortized Cost	164,542	206,356
Available-for-sale, Allowance for Credit Loss	0	0
Total available for sale, Gross Unrealized Gains	8,585	8,755
Total available for sale, Gross Unrealized Losses	(788)	(638)
Total available for sale, Fair Value	172,339	214,473
Total available for sale, Carrying Value	172,339	214,473
Local general obligation
Debt Securities, Available-for-sale [Line Items]
Total available for sale, Amortized Cost	432,788	410,583
Available-for-sale, Allowance for Credit Loss	0	0
Total available for sale, Gross Unrealized Gains	34,850	40,596
Total available for sale, Gross Unrealized Losses	(585)	(555)
Total available for sale, Fair Value	467,053	450,624
Total available for sale, Carrying Value	467,053	450,624
Commercial
Debt Securities, Available-for-sale [Line Items]
Total available for sale, Amortized Cost	172,817	181,105
Available-for-sale, Allowance for Credit Loss	0	0
Total available for sale, Gross Unrealized Gains	6,555	6,725
Total available for sale, Gross Unrealized Losses	(86)	(113)
Total available for sale, Fair Value	179,286	187,717
Total available for sale, Carrying Value	179,286	187,717
Mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Total available for sale, Amortized Cost	1,062,455	994,292
Available-for-sale, Allowance for Credit Loss	0	0
Total available for sale, Gross Unrealized Gains	25,870	31,389
Total available for sale, Gross Unrealized Losses	(12,301)	(5,351)
Total available for sale, Fair Value	1,076,024	1,020,330
Total available for sale, Carrying Value	1,076,024	1,020,330
Asset-backed
Debt Securities, Available-for-sale [Line Items]
Total available for sale, Amortized Cost	3,952,009	3,218,048
Available-for-sale, Allowance for Credit Loss	0	0
Total available for sale, Gross Unrealized Gains	10,789	10,035
Total available for sale, Gross Unrealized Losses	(18,160)	(33,497)
Total available for sale, Fair Value	3,944,638	3,194,586
Total available for sale, Carrying Value	3,944,638	3,194,586
Industrial
Debt Securities, Available-for-sale [Line Items]
Total available for sale, Amortized Cost	2,986,688	2,456,516
Available-for-sale, Allowance for Credit Loss	(16)	(518)
Total available for sale, Gross Unrealized Gains	88,810	115,926
Total available for sale, Gross Unrealized Losses	(14,819)	(7,449)
Total available for sale, Fair Value	3,060,663	2,564,475
Total available for sale, Carrying Value	3,060,663	2,564,475
Financial
Debt Securities, Available-for-sale [Line Items]
Total available for sale, Amortized Cost	1,517,251	1,513,943
Available-for-sale, Allowance for Credit Loss	0	0
Total available for sale, Gross Unrealized Gains	48,756	62,947
Total available for sale, Gross Unrealized Losses	(1,638)	(987)
Total available for sale, Fair Value	1,564,369	1,575,903
Total available for sale, Carrying Value	1,564,369	1,575,903
Utilities
Debt Securities, Available-for-sale [Line Items]
Total available for sale, Amortized Cost	426,213	389,267
Available-for-sale, Allowance for Credit Loss	0	0
Total available for sale, Gross Unrealized Gains	15,476	31,931
Total available for sale, Gross Unrealized Losses	(2,360)	(33)
Total available for sale, Fair Value	439,329	421,165
Total available for sale, Carrying Value	439,329	421,165
Other
Debt Securities, Available-for-sale [Line Items]
Total available for sale, Amortized Cost	153,700	109,353
Available-for-sale, Allowance for Credit Loss	0	0
Total available for sale, Gross Unrealized Gains	429	696
Total available for sale, Gross Unrealized Losses	(301)	(11)
Total available for sale, Fair Value	153,828	110,038
Total available for sale, Carrying Value	153,828	110,038
Corporate
Debt Securities, Available-for-sale [Line Items]
Total available for sale, Amortized Cost	5,083,852	4,469,079
Available-for-sale, Allowance for Credit Loss	(16)	(518)	(6,436)	0
Total available for sale, Gross Unrealized Gains	153,471	211,500
Total available for sale, Gross Unrealized Losses	(19,118)	(8,480)
Total available for sale, Fair Value	5,218,189	4,671,581
Total available for sale, Carrying Value	5,218,189	4,671,581
Foreign government
Debt Securities, Available-for-sale [Line Items]
Total available for sale, Amortized Cost	1,072,930	993,268
Available-for-sale, Allowance for Credit Loss	(19,993)	(1,264)	$ (60,920)	$ 0
Total available for sale, Gross Unrealized Gains	19,361	28,007
Total available for sale, Gross Unrealized Losses	(42,419)	(44,448)
Total available for sale, Fair Value	1,029,879	975,563
Total available for sale, Carrying Value	$ 1,029,879	$ 975,563