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Investments (Gross Unrealized Losses on Investments, By Category and Length of Time) (Details) - USD ($)
$ in Thousands
Jun. 30, 2017
Dec. 31, 2016
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]    
Available for sale securities, continuous unrealized loss position, less than 12 months, fair value $ 7,273,237 $ 13,195,027
Available for sale securities, continuous unrealized loss position, less than 12 months, unrealized losses (201,792) (454,013)
Available for sale securities, continuous unrealized loss position, 12 months or longer, fair value 1,565,737 1,368,974
Available for sale securities, continuous unrealized loss position, 12 months or longer, unrealized losses (116,453) (130,705)
Available for sale securities, continuous unrealized loss position, total, fair value 8,838,974 14,564,001
Available for sale securities, continuous unrealized loss position, total, unrealized losses (318,245) (584,718)
United States Government Full Faith and Credit [Member]    
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]    
Available for sale securities, continuous unrealized loss position, less than 12 months, fair value 6,867 7,405
Available for sale securities, continuous unrealized loss position, less than 12 months, unrealized losses (168) (288)
Available for sale securities, continuous unrealized loss position, 12 months or longer, fair value 0 0
Available for sale securities, continuous unrealized loss position, 12 months or longer, unrealized losses 0 0
Available for sale securities, continuous unrealized loss position, total, fair value 6,867 7,405
Available for sale securities, continuous unrealized loss position, total, unrealized losses (168) (288)
United States Government Sponsored Agencies [Member]    
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]    
Available for sale securities, continuous unrealized loss position, less than 12 months, fair value 945,126 995,548
Available for sale securities, continuous unrealized loss position, less than 12 months, unrealized losses (35,343) (46,913)
Available for sale securities, continuous unrealized loss position, 12 months or longer, fair value 49,503 0
Available for sale securities, continuous unrealized loss position, 12 months or longer, unrealized losses (2,497) 0
Available for sale securities, continuous unrealized loss position, total, fair value 994,629 995,548
Available for sale securities, continuous unrealized loss position, total, unrealized losses (37,840) (46,913)
United States Municipalities, States and Territories [Member]    
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]    
Available for sale securities, continuous unrealized loss position, less than 12 months, fair value 273,302 463,409
Available for sale securities, continuous unrealized loss position, less than 12 months, unrealized losses (9,572) (22,393)
Available for sale securities, continuous unrealized loss position, 12 months or longer, fair value 0 0
Available for sale securities, continuous unrealized loss position, 12 months or longer, unrealized losses 0 0
Available for sale securities, continuous unrealized loss position, total, fair value 273,302 463,409
Available for sale securities, continuous unrealized loss position, total, unrealized losses (9,572) (22,393)
Foreign Government Obligations [Member]    
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]    
Available for sale securities, continuous unrealized loss position, less than 12 months, fair value 0 29,158
Available for sale securities, continuous unrealized loss position, less than 12 months, unrealized losses 0 (913)
Available for sale securities, continuous unrealized loss position, 12 months or longer, fair value 11,844 20,388
Available for sale securities, continuous unrealized loss position, 12 months or longer, unrealized losses (2,489) (4,167)
Available for sale securities, continuous unrealized loss position, total, fair value 11,844 49,546
Available for sale securities, continuous unrealized loss position, total, unrealized losses (2,489) (5,080)
Corporate Securities [Member] | Finance, Insurance and Real Estate Sectors [Member]    
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]    
Available for sale securities, continuous unrealized loss position, less than 12 months, fair value 1,132,134 1,940,107
Available for sale securities, continuous unrealized loss position, less than 12 months, unrealized losses (37,599) (70,421)
Available for sale securities, continuous unrealized loss position, 12 months or longer, fair value 86,335 82,907
Available for sale securities, continuous unrealized loss position, 12 months or longer, unrealized losses (1,495) (7,723)
Available for sale securities, continuous unrealized loss position, total, fair value 1,218,469 2,023,014
Available for sale securities, continuous unrealized loss position, total, unrealized losses (39,094) (78,144)
Corporate Securities [Member] | Manufacturing, Construction and Mining [Member]    
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]    
Available for sale securities, continuous unrealized loss position, less than 12 months, fair value 458,458 1,199,420
Available for sale securities, continuous unrealized loss position, less than 12 months, unrealized losses (9,156) (34,304)
Available for sale securities, continuous unrealized loss position, 12 months or longer, fair value 200,709 311,591
Available for sale securities, continuous unrealized loss position, 12 months or longer, unrealized losses (13,648) (23,273)
Available for sale securities, continuous unrealized loss position, total, fair value 659,167 1,511,011
Available for sale securities, continuous unrealized loss position, total, unrealized losses (22,804) (57,577)
Corporate Securities [Member] | Utilities and Related Sectors [Member]    
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]    
Available for sale securities, continuous unrealized loss position, less than 12 months, fair value 523,863 1,401,650
Available for sale securities, continuous unrealized loss position, less than 12 months, unrealized losses (14,319) (45,015)
Available for sale securities, continuous unrealized loss position, 12 months or longer, fair value 43,553 58,597
Available for sale securities, continuous unrealized loss position, 12 months or longer, unrealized losses (2,612) (5,820)
Available for sale securities, continuous unrealized loss position, total, fair value 567,416 1,460,247
Available for sale securities, continuous unrealized loss position, total, unrealized losses (16,931) (50,835)
Corporate Securities [Member] | Wholesale/Retail Trade [Member]    
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]    
Available for sale securities, continuous unrealized loss position, less than 12 months, fair value 282,589 637,121
Available for sale securities, continuous unrealized loss position, less than 12 months, unrealized losses (9,583) (18,880)
Available for sale securities, continuous unrealized loss position, 12 months or longer, fair value 42,120 29,719
Available for sale securities, continuous unrealized loss position, 12 months or longer, unrealized losses (4,812) (1,930)
Available for sale securities, continuous unrealized loss position, total, fair value 324,709 666,840
Available for sale securities, continuous unrealized loss position, total, unrealized losses (14,395) (20,810)
Corporate Securities [Member] | Services, Media and Other [Member]    
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]    
Available for sale securities, continuous unrealized loss position, less than 12 months, fair value 1,156,695 2,539,519
Available for sale securities, continuous unrealized loss position, less than 12 months, unrealized losses (33,573) (82,196)
Available for sale securities, continuous unrealized loss position, 12 months or longer, fair value 619,260 716,857
Available for sale securities, continuous unrealized loss position, 12 months or longer, unrealized losses (56,357) (79,211)
Available for sale securities, continuous unrealized loss position, total, fair value 1,775,955 3,256,376
Available for sale securities, continuous unrealized loss position, total, unrealized losses (89,930) (161,407)
Corporate Securities [Member] | Insurance [Member]    
Held-to-maturity Securities, Continuous Unrealized Loss Position [Abstract]    
Held for investment securities, continuous unrealized loss position, less than 12 months, fair value 0 0
Held for investment securities, continuous unrealized loss position, less than 12 months, unrealized losses 0 0
Held for investment securities, continuous unrealized loss position, 12 months or longer, fair value 76,702 68,766
Held for investment securities, continuous unrealized loss position, 12 months or longer, unrealized losses (229) (8,059)
Held for investment securities, continuous unrealized loss position, total, fair value 76,702 68,766
Held for investment securities, continuous unrealized loss position, total, unrealized losses (229) (8,059)
Residential Mortgage Backed Securities [Member]    
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]    
Available for sale securities, continuous unrealized loss position, less than 12 months, fair value 40,684 81,762
Available for sale securities, continuous unrealized loss position, less than 12 months, unrealized losses (1,631) (3,463)
Available for sale securities, continuous unrealized loss position, 12 months or longer, fair value 8,036 1,853
Available for sale securities, continuous unrealized loss position, 12 months or longer, unrealized losses (681) (91)
Available for sale securities, continuous unrealized loss position, total, fair value 48,720 83,615
Available for sale securities, continuous unrealized loss position, total, unrealized losses (2,312) (3,554)
Commercial Mortgage Backed Securities [Member]    
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]    
Available for sale securities, continuous unrealized loss position, less than 12 months, fair value 1,924,949 3,148,395
Available for sale securities, continuous unrealized loss position, less than 12 months, unrealized losses (44,461) (116,938)
Available for sale securities, continuous unrealized loss position, 12 months or longer, fair value 439,081 895
Available for sale securities, continuous unrealized loss position, 12 months or longer, unrealized losses (24,788) (76)
Available for sale securities, continuous unrealized loss position, total, fair value 2,364,030 3,149,290
Available for sale securities, continuous unrealized loss position, total, unrealized losses (69,249) (117,014)
Other Asset Backed Securities [Member]    
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]    
Available for sale securities, continuous unrealized loss position, less than 12 months, fair value 528,570 751,533
Available for sale securities, continuous unrealized loss position, less than 12 months, unrealized losses (6,387) (12,289)
Available for sale securities, continuous unrealized loss position, 12 months or longer, fair value 65,296 146,167
Available for sale securities, continuous unrealized loss position, 12 months or longer, unrealized losses (7,074) (8,414)
Available for sale securities, continuous unrealized loss position, total, fair value 593,866 897,700
Available for sale securities, continuous unrealized loss position, total, unrealized losses $ (13,461) $ (20,703)