Fixed Maturity Securities In An Unrealized Loss Position (Securities in an Unrealized Loss Position) (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Debt securities
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than 12 Months	$ 1,384,645	$ 4,895,573
Gross Unrealized Losses, Less than 12 Months	38,434	118,061
Fair Value, 12 Months or Greater	1,672,556	2,965,937
Gross Unrealized Losses, 12 Months or Greater	37,311	97,502
Fair Value, Total	3,057,201	7,861,510
Gross Unrealized Losses, Total	75,745	215,563
U.S. government and government agency
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than 12 Months	6,175	195,359
Gross Unrealized Losses, Less than 12 Months	37	933
Fair Value, 12 Months or Greater	92,729	130,815
Gross Unrealized Losses, 12 Months or Greater	1,381	3,977
Fair Value, Total	98,904	326,174
Gross Unrealized Losses, Total	1,418	4,910
State and municipal
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than 12 Months	64,499	701,700
Gross Unrealized Losses, Less than 12 Months	838	6,874
Fair Value, 12 Months or Greater	270,454	744,905
Gross Unrealized Losses, 12 Months or Greater	3,083	17,738
Fair Value, Total	334,953	1,446,605
Gross Unrealized Losses, Total	3,921	24,612
Mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than 12 Months	23,049	334,063
Gross Unrealized Losses, Less than 12 Months	112	2,911
Fair Value, 12 Months or Greater	334,887	712,595
Gross Unrealized Losses, 12 Months or Greater	5,584	21,022
Fair Value, Total	357,936	1,046,658
Gross Unrealized Losses, Total	5,696	23,933
Asset-backed
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than 12 Months	1,010,706	1,687,665
Gross Unrealized Losses, Less than 12 Months	17,410	28,965
Fair Value, 12 Months or Greater	572,110	342,855
Gross Unrealized Losses, 12 Months or Greater	10,297	4,722
Fair Value, Total	1,582,816	2,030,520
Gross Unrealized Losses, Total	27,707	33,687
Corporate
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than 12 Months	139,627	1,730,513
Gross Unrealized Losses, Less than 12 Months	1,095	54,181
Fair Value, 12 Months or Greater	328,409	954,763
Gross Unrealized Losses, 12 Months or Greater	12,549	49,129
Fair Value, Total	468,036	2,685,276
Gross Unrealized Losses, Total	13,644	103,310
Foreign government
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than 12 Months	140,589	246,273
Gross Unrealized Losses, Less than 12 Months	18,942	24,197
Fair Value, 12 Months or Greater	73,967	80,004
Gross Unrealized Losses, 12 Months or Greater	4,417	914
Fair Value, Total	214,556	326,277
Gross Unrealized Losses, Total	$ 23,359	$ 25,111