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Investments (Gross Unrealized Losses on Investments, By Category and Length of Time) (Details) - USD ($)
$ in Thousands
Dec. 31, 2017
Dec. 31, 2016
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]    
Available for sale, continuous unrealized loss position, less than 12 months, fair value $ 3,999,294 $ 13,195,027
Available for sale, continuous unrealized loss position, less than 12 months, unrealized losses (47,884) (454,013)
Available for sale, continuous unrealized loss position, 12 months or more, fair value 4,815,325 1,368,974
Available for sale, continuous unrealized loss position, 12 months or more, unrealized losses (221,017) (130,705)
Available for sale, continuous unrealized loss position, total, fair value 8,814,619 14,564,001
Available for sale, continuous unrealized loss position, total, unrealized losses (268,901) (584,718)
United States Government Full Faith and Credit [Member]    
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]    
Available for sale, continuous unrealized loss position, less than 12 months, fair value 1,565 7,405
Available for sale, continuous unrealized loss position, less than 12 months, unrealized losses (10) (288)
Available for sale, continuous unrealized loss position, 12 months or more, fair value 6,731 0
Available for sale, continuous unrealized loss position, 12 months or more, unrealized losses (137) 0
Available for sale, continuous unrealized loss position, total, fair value 8,296 7,405
Available for sale, continuous unrealized loss position, total, unrealized losses (147) (288)
United States Government Sponsored Agencies [Member]    
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]    
Available for sale, continuous unrealized loss position, less than 12 months, fair value 44,794 995,548
Available for sale, continuous unrealized loss position, less than 12 months, unrealized losses (180) (46,913)
Available for sale, continuous unrealized loss position, 12 months or more, fair value 958,965 0
Available for sale, continuous unrealized loss position, 12 months or more, unrealized losses (31,550) 0
Available for sale, continuous unrealized loss position, total, fair value 1,003,759 995,548
Available for sale, continuous unrealized loss position, total, unrealized losses (31,730) (46,913)
United States Municipalities, States and Territories [Member]    
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]    
Available for sale, continuous unrealized loss position, less than 12 months, fair value 44,736 463,409
Available for sale, continuous unrealized loss position, less than 12 months, unrealized losses (128) (22,393)
Available for sale, continuous unrealized loss position, 12 months or more, fair value 128,499 0
Available for sale, continuous unrealized loss position, 12 months or more, unrealized losses (3,468) 0
Available for sale, continuous unrealized loss position, total, fair value 173,235 463,409
Available for sale, continuous unrealized loss position, total, unrealized losses (3,596) (22,393)
Foreign Government Obligations [Member]    
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]    
Available for sale, continuous unrealized loss position, less than 12 months, fair value 49,663 29,158
Available for sale, continuous unrealized loss position, less than 12 months, unrealized losses (337) (913)
Available for sale, continuous unrealized loss position, 12 months or more, fair value 12,625 20,388
Available for sale, continuous unrealized loss position, 12 months or more, unrealized losses (1,688) (4,167)
Available for sale, continuous unrealized loss position, total, fair value 62,288 49,546
Available for sale, continuous unrealized loss position, total, unrealized losses (2,025) (5,080)
Corporate Securities [Member] | Finance, Insurance and Real Estate [Member]    
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]    
Available for sale, continuous unrealized loss position, less than 12 months, fair value 456,244 1,940,107
Available for sale, continuous unrealized loss position, less than 12 months, unrealized losses (5,135) (70,421)
Available for sale, continuous unrealized loss position, 12 months or more, fair value 600,655 82,907
Available for sale, continuous unrealized loss position, 12 months or more, unrealized losses (28,043) (7,723)
Available for sale, continuous unrealized loss position, total, fair value 1,056,899 2,023,014
Available for sale, continuous unrealized loss position, total, unrealized losses (33,178) (78,144)
Corporate Securities [Member] | Manufacturing, Construction and Mining [Member]    
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]    
Available for sale, continuous unrealized loss position, less than 12 months, fair value 222,985 1,199,420
Available for sale, continuous unrealized loss position, less than 12 months, unrealized losses (3,475) (34,304)
Available for sale, continuous unrealized loss position, 12 months or more, fair value 231,196 311,591
Available for sale, continuous unrealized loss position, 12 months or more, unrealized losses (10,849) (23,273)
Available for sale, continuous unrealized loss position, total, fair value 454,181 1,511,011
Available for sale, continuous unrealized loss position, total, unrealized losses (14,324) (57,577)
Corporate Securities [Member] | Utilities and Related Sectors [Member]    
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]    
Available for sale, continuous unrealized loss position, less than 12 months, fair value 395,183 1,401,650
Available for sale, continuous unrealized loss position, less than 12 months, unrealized losses (4,099) (45,015)
Available for sale, continuous unrealized loss position, 12 months or more, fair value 249,416 58,597
Available for sale, continuous unrealized loss position, 12 months or more, unrealized losses (8,901) (5,820)
Available for sale, continuous unrealized loss position, total, fair value 644,599 1,460,247
Available for sale, continuous unrealized loss position, total, unrealized losses (13,000) (50,835)
Corporate Securities [Member] | Wholesale/Retail Trade [Member]    
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]    
Available for sale, continuous unrealized loss position, less than 12 months, fair value 152,941 637,121
Available for sale, continuous unrealized loss position, less than 12 months, unrealized losses (1,249) (18,880)
Available for sale, continuous unrealized loss position, 12 months or more, fair value 178,635 29,719
Available for sale, continuous unrealized loss position, 12 months or more, unrealized losses (11,371) (1,930)
Available for sale, continuous unrealized loss position, total, fair value 331,576 666,840
Available for sale, continuous unrealized loss position, total, unrealized losses (12,620) (20,810)
Corporate Securities [Member] | Services, Media and Other [Member]    
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]    
Available for sale, continuous unrealized loss position, less than 12 months, fair value 729,124 2,539,519
Available for sale, continuous unrealized loss position, less than 12 months, unrealized losses (19,000) (82,196)
Available for sale, continuous unrealized loss position, 12 months or more, fair value 891,654 716,857
Available for sale, continuous unrealized loss position, 12 months or more, unrealized losses (53,565) (79,211)
Available for sale, continuous unrealized loss position, total, fair value 1,620,778 3,256,376
Available for sale, continuous unrealized loss position, total, unrealized losses (72,565) (161,407)
Corporate Securities [Member] | Insurance [Member]    
Held-to-maturity Securities, Continuous Unrealized Loss Position [Abstract]    
Held for investment, continuous unrealized loss position, less than 12 months, fair value 0 0
Held for investment, continuous unrealized loss position, less than 12 months, unrealized losses 0 0
Held for investment, continuous unrealized loss position, 12 months or more, fair value 76,460 68,766
Held for investment, continuous unrealized loss position, 12 months or more, unrealized losses (581) (8,059)
Held for investment, continuous unrealized loss position, total, fair value 76,460 68,766
Held for investment, continuous unrealized loss position, total, unrealized losses (581) (8,059)
Residential Mortgage Backed Securities [Member]    
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]    
Available for sale, continuous unrealized loss position, less than 12 months, fair value 39,771 81,762
Available for sale, continuous unrealized loss position, less than 12 months, unrealized losses (387) (3,463)
Available for sale, continuous unrealized loss position, 12 months or more, fair value 32,917 1,853
Available for sale, continuous unrealized loss position, 12 months or more, unrealized losses (2,084) (91)
Available for sale, continuous unrealized loss position, total, fair value 72,688 83,615
Available for sale, continuous unrealized loss position, total, unrealized losses (2,471) (3,554)
Commercial Mortgage Backed Securities [Member]    
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]    
Available for sale, continuous unrealized loss position, less than 12 months, fair value 1,096,757 3,148,395
Available for sale, continuous unrealized loss position, less than 12 months, unrealized losses (10,385) (116,938)
Available for sale, continuous unrealized loss position, 12 months or more, fair value 1,306,437 895
Available for sale, continuous unrealized loss position, 12 months or more, unrealized losses (59,455) (76)
Available for sale, continuous unrealized loss position, total, fair value 2,403,194 3,149,290
Available for sale, continuous unrealized loss position, total, unrealized losses (69,840) (117,014)
Other Asset Backed Securities [Member]    
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]    
Available for sale, continuous unrealized loss position, less than 12 months, fair value 765,531 751,533
Available for sale, continuous unrealized loss position, less than 12 months, unrealized losses (3,499) (12,289)
Available for sale, continuous unrealized loss position, 12 months or more, fair value 217,595 146,167
Available for sale, continuous unrealized loss position, 12 months or more, unrealized losses (9,906) (8,414)
Available for sale, continuous unrealized loss position, total, fair value 983,126 897,700
Available for sale, continuous unrealized loss position, total, unrealized losses $ (13,405) $ (20,703)