Investments (Schedule of Fixed Maturity and Equity Securities) (Details) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Investment Holdings [Line Items]
Held for investment, amortized cost	$ 76,622	$ 76,432
Held for investment, fair value	65,377	75,838
Fixed Maturity Securities [Member] \| Available For Sale [Member]
Investment Holdings [Line Items]
Available for sale securities, amortized cost	35,823,710	30,205,046
Available for sale securities, gross unrealized gains	1,443,474	2,395,577
Available for sale securities, gross unrealized losses	(845,345)	(155,421)
Available for sale securities, fair value	36,421,839	32,445,202
Equity Securities [Member] \| Available For Sale [Member]
Investment Holdings [Line Items]
Available for sale securities, amortized cost	7,515	7,509
Available for sale securities, gross unrealized gains	313	296
Available for sale securities, gross unrealized losses	0	0
Available for sale securities, fair value	7,828	7,805
United States Government Full Faith and Credit [Member] \| Fixed Maturity Securities [Member] \| Available For Sale [Member]
Investment Holdings [Line Items]
Available for sale securities, amortized cost	470,567	137,710
Available for sale securities, gross unrealized gains	988	765
Available for sale securities, gross unrealized losses	(299)	(15)
Available for sale securities, fair value	471,256	138,460
United States Government Sponsored Agencies [Member] \| Fixed Maturity Securities [Member] \| Available For Sale [Member]
Investment Holdings [Line Items]
Available for sale securities, amortized cost	1,386,219	1,364,424
Available for sale securities, gross unrealized gains	26,801	43,399
Available for sale securities, gross unrealized losses	(14,409)	(13,933)
Available for sale securities, fair value	1,398,611	1,393,890
United States Municipalities, States and Territories [Member] \| Fixed Maturity Securities [Member] \| Available For Sale [Member]
Investment Holdings [Line Items]
Available for sale securities, amortized cost	3,422,667	3,293,551
Available for sale securities, gross unrealized gains	341,328	430,469
Available for sale securities, gross unrealized losses	(8,628)	(711)
Available for sale securities, fair value	3,755,367	3,723,309
Foreign Government Obligations [Member] \| Fixed Maturity Securities [Member] \| Available For Sale [Member]
Investment Holdings [Line Items]
Available for sale securities, amortized cost	210,953	181,128
Available for sale securities, gross unrealized gains	12,547	16,628
Available for sale securities, gross unrealized losses	(10,935)	(3,953)
Available for sale securities, fair value	212,565	193,803
Corporate Securities [Member] \| Fixed Maturity Securities [Member] \| Available For Sale [Member]
Investment Holdings [Line Items]
Available for sale securities, amortized cost	23,597,530	19,984,747
Available for sale securities, gross unrealized gains	887,288	1,628,941
Available for sale securities, gross unrealized losses	(682,424)	(123,396)
Available for sale securities, fair value	23,802,394	21,490,292
Corporate Securities [Member] \| Fixed Maturity Securities [Member] \| Held For Investment [Member]
Investment Holdings [Line Items]
Held for investment, amortized cost	76,622	76,432
Held for investment, gross unrealized gains	0	0
Held for investment, gross unrealized losses	(11,245)	(594)
Held for investment, fair value	65,377	75,838
Residential Mortgage Backed Securities [Member] \| Fixed Maturity Securities [Member] \| Available For Sale [Member]
Investment Holdings [Line Items]
Available for sale securities, amortized cost	1,366,985	1,616,846
Available for sale securities, gross unrealized gains	98,576	136,704
Available for sale securities, gross unrealized losses	(3,489)	(2,205)
Available for sale securities, fair value	1,462,072	1,751,345
Commercial Mortgage Backed Securities [Member] \| Fixed Maturity Securities [Member] \| Available For Sale [Member]
Investment Holdings [Line Items]
Available for sale securities, amortized cost	4,238,265	2,720,294
Available for sale securities, gross unrealized gains	41,412	90,649
Available for sale securities, gross unrealized losses	(105,281)	(3,323)
Available for sale securities, fair value	4,174,396	2,807,620
Other Asset Backed Securities [Member] \| Fixed Maturity Securities [Member] \| Available For Sale [Member]
Investment Holdings [Line Items]
Available for sale securities, amortized cost	1,130,524	906,346
Available for sale securities, gross unrealized gains	34,534	48,022
Available for sale securities, gross unrealized losses	(19,880)	(7,885)
Available for sale securities, fair value	$ 1,145,178	$ 946,483

Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.

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