Securities in an Unrealized Loss Position (Securities in an Unrealized Loss Position) (Details) - External Credit Rating, Investment Grade - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
U.S. government and government agency
Debt Securities, Available-for-sale [Line Items]
Available-for-sale Securities, Less Than 12 Months, Fair Value	$ 83,837	$ 195,359
Available-for-sale Securities, Less Than 12 Months, Gross Unrealized Losses	618	933
Available-for-sale Securities, 12 Months or Greater, Fair Value	53,089	130,815
Available-for-sale Securities, 12 Months or Greater, Gross Unrealized Losses	857	3,977
Available-for-sale Securities, Fair Value	136,926	326,174
Available For Sale Securities, Gross Unrealized Losses	1,475	4,910
State and municipal
Debt Securities, Available-for-sale [Line Items]
Available-for-sale Securities, Less Than 12 Months, Fair Value	365,184	701,700
Available-for-sale Securities, Less Than 12 Months, Gross Unrealized Losses	4,245	6,874
Available-for-sale Securities, 12 Months or Greater, Fair Value	127,210	744,905
Available-for-sale Securities, 12 Months or Greater, Gross Unrealized Losses	1,682	17,738
Available-for-sale Securities, Fair Value	492,394	1,446,605
Available For Sale Securities, Gross Unrealized Losses	5,927	24,612
Mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Available-for-sale Securities, Less Than 12 Months, Fair Value	301,358	334,063
Available-for-sale Securities, Less Than 12 Months, Gross Unrealized Losses	2,281	2,911
Available-for-sale Securities, 12 Months or Greater, Fair Value	180,148	712,595
Available-for-sale Securities, 12 Months or Greater, Gross Unrealized Losses	3,220	21,022
Available-for-sale Securities, Fair Value	481,506	1,046,658
Available For Sale Securities, Gross Unrealized Losses	5,501	23,933
Asset-backed securities
Debt Securities, Available-for-sale [Line Items]
Available-for-sale Securities, Less Than 12 Months, Fair Value	755,259	1,687,665
Available-for-sale Securities, Less Than 12 Months, Gross Unrealized Losses	2,307	28,965
Available-for-sale Securities, 12 Months or Greater, Fair Value	774,508	342,855
Available-for-sale Securities, 12 Months or Greater, Gross Unrealized Losses	19,183	4,722
Available-for-sale Securities, Fair Value	1,529,767	2,030,520
Available For Sale Securities, Gross Unrealized Losses	21,490	33,687
Total corporate
Debt Securities, Available-for-sale [Line Items]
Available-for-sale Securities, Less Than 12 Months, Fair Value	307,367	1,730,513
Available-for-sale Securities, Less Than 12 Months, Gross Unrealized Losses	3,148	54,181
Available-for-sale Securities, 12 Months or Greater, Fair Value	121,470	954,763
Available-for-sale Securities, 12 Months or Greater, Gross Unrealized Losses	5,172	49,129
Available-for-sale Securities, Fair Value	428,837	2,685,276
Available For Sale Securities, Gross Unrealized Losses	8,320	103,310
Foreign government
Debt Securities, Available-for-sale [Line Items]
Available-for-sale Securities, Less Than 12 Months, Fair Value	164,536	246,273
Available-for-sale Securities, Less Than 12 Months, Gross Unrealized Losses	32,028	24,197
Available-for-sale Securities, 12 Months or Greater, Fair Value	107,266	80,004
Available-for-sale Securities, 12 Months or Greater, Gross Unrealized Losses	61,645	914
Available-for-sale Securities, Fair Value	271,802	326,277
Available For Sale Securities, Gross Unrealized Losses	93,673	25,111
Fixed maturity securities
Debt Securities, Available-for-sale [Line Items]
Available-for-sale Securities, Less Than 12 Months, Fair Value	1,977,541	4,895,573
Available-for-sale Securities, Less Than 12 Months, Gross Unrealized Losses	44,627	118,061
Available-for-sale Securities, 12 Months or Greater, Fair Value	1,363,691	2,965,937
Available-for-sale Securities, 12 Months or Greater, Gross Unrealized Losses	91,759	97,502
Available-for-sale Securities, Fair Value	3,341,232	7,861,510
Available For Sale Securities, Gross Unrealized Losses	$ 136,386	$ 215,563