Fixed Maturity Securities In An Unrealized Loss Position (Securities in an Unrealized Loss Position) (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Debt securities
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than 12 Months	$ 5,279,249	$ 1,977,541
Gross Unrealized Losses, Less than 12 Months	236,721	44,627
Fair Value, 12 Months or Greater	862,461	1,363,691
Gross Unrealized Losses, 12 Months or Greater	124,656	91,759
Fair Value, Total	6,141,710	3,341,232
Gross Unrealized Losses, Total	361,377	136,386
U.S. government and government agency
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than 12 Months	13,308	83,837
Gross Unrealized Losses, Less than 12 Months	13	618
Fair Value, 12 Months or Greater	7,145	53,089
Gross Unrealized Losses, 12 Months or Greater	45	857
Fair Value, Total	20,453	136,926
Gross Unrealized Losses, Total	58	1,475
State and municipal
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than 12 Months	640,570	365,184
Gross Unrealized Losses, Less than 12 Months	11,306	4,245
Fair Value, 12 Months or Greater	74,998	127,210
Gross Unrealized Losses, 12 Months or Greater	1,673	1,682
Fair Value, Total	715,568	492,394
Gross Unrealized Losses, Total	12,979	5,927
Mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than 12 Months	609,582	301,358
Gross Unrealized Losses, Less than 12 Months	24,143	2,281
Fair Value, 12 Months or Greater	86,569	180,148
Gross Unrealized Losses, 12 Months or Greater	1,025	3,220
Fair Value, Total	696,151	481,506
Gross Unrealized Losses, Total	25,168	5,501
Asset-backed
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than 12 Months	2,014,456	755,259
Gross Unrealized Losses, Less than 12 Months	108,100	2,307
Fair Value, 12 Months or Greater	614,876	774,508
Gross Unrealized Losses, 12 Months or Greater	79,890	19,183
Fair Value, Total	2,629,332	1,529,767
Gross Unrealized Losses, Total	187,990	21,490
Corporate
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than 12 Months	1,886,894	307,367
Gross Unrealized Losses, Less than 12 Months	85,902	3,148
Fair Value, 12 Months or Greater	54,226	121,470
Gross Unrealized Losses, 12 Months or Greater	6,443	5,172
Fair Value, Total	1,941,120	428,837
Gross Unrealized Losses, Total	92,345	8,320
Foreign government
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than 12 Months	114,439	164,536
Gross Unrealized Losses, Less than 12 Months	7,257	32,028
Fair Value, 12 Months or Greater	24,647	107,266
Gross Unrealized Losses, 12 Months or Greater	35,580	61,645
Fair Value, Total	139,086	271,802
Gross Unrealized Losses, Total	$ 42,837	$ 93,673