Fixed Maturity Securities In An Unrealized Loss Position (Securities in an Unrealized Loss Position) (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Debt securities
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than 12 Months	$ 1,869,549	$ 4,895,573
Gross Unrealized Losses, Less than 12 Months	49,081	118,061
Fair Value, 12 Months or Greater	3,468,525	2,965,937
Gross Unrealized Losses, 12 Months or Greater	62,074	97,502
Fair Value, Total	5,338,074	7,861,510
Gross Unrealized Losses, Total	111,155	215,563
U.S. government and government agency
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than 12 Months	97,181	195,359
Gross Unrealized Losses, Less than 12 Months	94	933
Fair Value, 12 Months or Greater	113,805	130,815
Gross Unrealized Losses, 12 Months or Greater	2,840	3,977
Fair Value, Total	210,986	326,174
Gross Unrealized Losses, Total	2,934	4,910
State and municipal
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than 12 Months	70,022	701,700
Gross Unrealized Losses, Less than 12 Months	994	6,874
Fair Value, 12 Months or Greater	658,166	744,905
Gross Unrealized Losses, 12 Months or Greater	7,110	17,738
Fair Value, Total	728,188	1,446,605
Gross Unrealized Losses, Total	8,104	24,612
Mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than 12 Months	36,125	334,063
Gross Unrealized Losses, Less than 12 Months	221	2,911
Fair Value, 12 Months or Greater	734,163	712,595
Gross Unrealized Losses, 12 Months or Greater	14,310	21,022
Fair Value, Total	770,288	1,046,658
Gross Unrealized Losses, Total	14,531	23,933
Asset-backed
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than 12 Months	1,250,397	1,687,665
Gross Unrealized Losses, Less than 12 Months	21,848	28,965
Fair Value, 12 Months or Greater	598,253	342,855
Gross Unrealized Losses, 12 Months or Greater	11,832	4,722
Fair Value, Total	1,848,650	2,030,520
Gross Unrealized Losses, Total	33,680	33,687
Corporate
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than 12 Months	223,109	1,730,513
Gross Unrealized Losses, Less than 12 Months	3,366	54,181
Fair Value, 12 Months or Greater	1,293,393	954,763
Gross Unrealized Losses, 12 Months or Greater	24,608	49,129
Fair Value, Total	1,516,502	2,685,276
Gross Unrealized Losses, Total	27,974	103,310
Foreign government
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than 12 Months	192,715	246,273
Gross Unrealized Losses, Less than 12 Months	22,558	24,197
Fair Value, 12 Months or Greater	70,745	80,004
Gross Unrealized Losses, 12 Months or Greater	1,374	914
Fair Value, Total	263,460	326,277
Gross Unrealized Losses, Total	$ 23,932	$ 25,111