Investments (Gross Unrealized Losses on Investments, By Category and Length of Time) (Details) - Fixed Maturity Securities [Member] - USD ($) $ in Thousands	Sep. 30, 2016	Dec. 31, 2015
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Available for sale securities, continuous unrealized loss position, less than 12 months, fair value	$ 2,426,891	$ 13,944,810
Available for sale securities, continuous unrealized loss position, less than 12 months, unrealized losses	(58,282)	(646,288)
Available for sale securities, continuous unrealized loss position, 12 months or longer, fair value	1,400,531	676,956
Available for sale securities, continuous unrealized loss position, 12 months or longer, unrealized losses	(125,364)	(199,057)
Available for sale securities, continuous unrealized loss position, total, fair value	3,827,422	14,621,766
Available for sale securities, continuous unrealized loss position, total, unrealized losses	(183,646)	(845,345)
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		37,730
Available for sale securities, continuous unrealized loss position, less than 12 months, unrealized losses		(299)
Available for sale securities, continuous unrealized loss position, 12 months or longer, fair value		0
Available for sale securities, continuous unrealized loss position, 12 months or longer, unrealized losses		0
Available for sale securities, continuous unrealized loss position, total, fair value		37,730
Available for sale securities, continuous unrealized loss position, total, unrealized losses		(299)
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	76,581	957,053
Available for sale securities, continuous unrealized loss position, less than 12 months, unrealized losses	(419)	(14,409)
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	76,581	957,053
Available for sale securities, continuous unrealized loss position, total, unrealized losses	(419)	(14,409)
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	68,084	261,823
Available for sale securities, continuous unrealized loss position, less than 12 months, unrealized losses	(368)	(8,474)
Available for sale securities, continuous unrealized loss position, 12 months or longer, fair value	0	2,846
Available for sale securities, continuous unrealized loss position, 12 months or longer, unrealized losses	0	(154)
Available for sale securities, continuous unrealized loss position, total, fair value	68,084	264,669
Available for sale securities, continuous unrealized loss position, total, unrealized losses	(368)	(8,628)
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	42,966
Available for sale securities, continuous unrealized loss position, less than 12 months, unrealized losses	0	(1,762)
Available for sale securities, continuous unrealized loss position, 12 months or longer, fair value	20,750	15,463
Available for sale securities, continuous unrealized loss position, 12 months or longer, unrealized losses	(3,825)	(9,173)
Available for sale securities, continuous unrealized loss position, total, fair value	20,750	58,429
Available for sale securities, continuous unrealized loss position, total, unrealized losses	(3,825)	(10,935)
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	453,189	2,077,223
Available for sale securities, continuous unrealized loss position, less than 12 months, unrealized losses	(5,447)	(59,607)
Available for sale securities, continuous unrealized loss position, 12 months or longer, fair value	100,261	49,912
Available for sale securities, continuous unrealized loss position, 12 months or longer, unrealized losses	(5,376)	(14,855)
Available for sale securities, continuous unrealized loss position, total, fair value	553,450	2,127,135
Available for sale securities, continuous unrealized loss position, total, unrealized losses	(10,823)	(74,462)
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	271,769	3,517,967
Available for sale securities, continuous unrealized loss position, less than 12 months, unrealized losses	(5,102)	(246,456)
Available for sale securities, continuous unrealized loss position, 12 months or longer, fair value	764,319	376,229
Available for sale securities, continuous unrealized loss position, 12 months or longer, unrealized losses	(80,318)	(131,003)
Available for sale securities, continuous unrealized loss position, total, fair value	1,036,088	3,894,196
Available for sale securities, continuous unrealized loss position, total, unrealized losses	(85,420)	(377,459)
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	157,473	2,240,652
Available for sale securities, continuous unrealized loss position, less than 12 months, unrealized losses	(3,440)	(138,940)
Available for sale securities, continuous unrealized loss position, 12 months or longer, fair value	221,059	97,184
Available for sale securities, continuous unrealized loss position, 12 months or longer, unrealized losses	(17,989)	(22,565)
Available for sale securities, continuous unrealized loss position, total, fair value	378,532	2,337,836
Available for sale securities, continuous unrealized loss position, total, unrealized losses	(21,429)	(161,505)
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	16,064	473,050
Available for sale securities, continuous unrealized loss position, less than 12 months, unrealized losses	(75)	(17,863)
Available for sale securities, continuous unrealized loss position, 12 months or longer, fair value	59,574	38,682
Available for sale securities, continuous unrealized loss position, 12 months or longer, unrealized losses	(3,914)	(8,125)
Available for sale securities, continuous unrealized loss position, total, fair value	75,638	511,732
Available for sale securities, continuous unrealized loss position, total, unrealized losses	(3,989)	(25,988)
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	136,667	1,037,011
Available for sale securities, continuous unrealized loss position, less than 12 months, unrealized losses	(4,545)	(39,937)
Available for sale securities, continuous unrealized loss position, 12 months or longer, fair value	133,607	32,050
Available for sale securities, continuous unrealized loss position, 12 months or longer, unrealized losses	(7,944)	(3,073)
Available for sale securities, continuous unrealized loss position, total, fair value	270,274	1,069,061
Available for sale securities, continuous unrealized loss position, total, unrealized losses	(12,489)	(43,010)
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	65,377
Held for investment securities, continuous unrealized loss position, less than 12 months, unrealized losses	0	(11,245)
Held for investment securities, continuous unrealized loss position, 12 months or longer, fair value	75,547	0
Held for investment securities, continuous unrealized loss position, 12 months or longer, unrealized losses	(1,226)	0
Held for investment securities, continuous unrealized loss position, total, fair value	75,547	65,377
Held for investment securities, continuous unrealized loss position, total, unrealized losses	(1,226)	(11,245)
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	38,200	162,770
Available for sale securities, continuous unrealized loss position, less than 12 months, unrealized losses	(2,144)	(2,958)
Available for sale securities, continuous unrealized loss position, 12 months or longer, fair value	1,976	6,438
Available for sale securities, continuous unrealized loss position, 12 months or longer, unrealized losses	(87)	(531)
Available for sale securities, continuous unrealized loss position, total, fair value	40,176	169,208
Available for sale securities, continuous unrealized loss position, total, unrealized losses	(2,231)	(3,489)
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	945,680	2,679,510
Available for sale securities, continuous unrealized loss position, less than 12 months, unrealized losses	(27,518)	(105,002)
Available for sale securities, continuous unrealized loss position, 12 months or longer, fair value	937	11,495
Available for sale securities, continuous unrealized loss position, 12 months or longer, unrealized losses	(33)	(279)
Available for sale securities, continuous unrealized loss position, total, fair value	946,617	2,691,005
Available for sale securities, continuous unrealized loss position, total, unrealized losses	(27,551)	(105,281)
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	263,184	457,055
Available for sale securities, continuous unrealized loss position, less than 12 months, unrealized losses	(9,224)	(10,581)
Available for sale securities, continuous unrealized loss position, 12 months or longer, fair value	98,048	46,657
Available for sale securities, continuous unrealized loss position, 12 months or longer, unrealized losses	(5,878)	(9,299)
Available for sale securities, continuous unrealized loss position, total, fair value	361,232	503,712
Available for sale securities, continuous unrealized loss position, total, unrealized losses	$ (15,102)	$ (19,880)