Investments (Schedule of Amortized Cost and Fair Value of Fixed Maturity and Equity Securities) (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Investment Holdings [Line Items]
Held for investment, amortized cost	$ 76,479us-gaap_HeldToMaturitySecurities	$ 76,432us-gaap_HeldToMaturitySecurities
Held for investment	82,984	75,838
Fixed Maturity Securities [Member] \| Available For Sale Securities [Member]
Investment Holdings [Line Items]
Available for sale, amortized cost	31,443,319	30,205,046
Available for sale, gross unrealized gains	2,870,024	2,395,577
Available for sale, gross unrealized losses	(109,702)	(155,421)
Available for sale, fair value	34,203,641	32,445,202
Equity Securities [Member] \| Available For Sale Securities [Member]
Investment Holdings [Line Items]
Available for sale, amortized cost	7,511	7,509
Available for sale, gross unrealized gains	338	296
Available for sale, gross unrealized losses	0	0
Available for sale, fair value	7,849	7,805
United States Government Full Faith and Credit [Member] \| Fixed Maturity Securities [Member] \| Available For Sale Securities [Member]
Investment Holdings [Line Items]
Available for sale, amortized cost	37,114	137,710
Available for sale, gross unrealized gains	1,417	765
Available for sale, gross unrealized losses	(4)	(15)
Available for sale, fair value	38,527	138,460
United States Government Sponsored Agencies [Member] \| Fixed Maturity Securities [Member] \| Available For Sale Securities [Member]
Investment Holdings [Line Items]
Available for sale, amortized cost	1,384,476	1,364,424
Available for sale, gross unrealized gains	45,559	43,399
Available for sale, gross unrealized losses	(8,671)	(13,933)
Available for sale, fair value	1,421,364	1,393,890
United States Municipalities, States and Territories [Member] \| Fixed Maturity Securities [Member] \| Available For Sale Securities [Member]
Investment Holdings [Line Items]
Available for sale, amortized cost	3,345,258	3,293,551
Available for sale, gross unrealized gains	464,981	430,469
Available for sale, gross unrealized losses	(377)	(711)
Available for sale, fair value	3,809,862	3,723,309
Foreign Government Obligations [Member] \| Fixed Maturity Securities [Member] \| Available For Sale Securities [Member]
Investment Holdings [Line Items]
Available for sale, amortized cost	210,918	181,128
Available for sale, gross unrealized gains	21,023	16,628
Available for sale, gross unrealized losses	(4,400)	(3,953)
Available for sale, fair value	227,541	193,803
Corporate Securities [Member] \| Fixed Maturity Securities [Member] \| Available For Sale Securities [Member]
Investment Holdings [Line Items]
Available for sale, amortized cost	20,864,311	19,984,747
Available for sale, gross unrealized gains	1,973,728	1,628,941
Available for sale, gross unrealized losses	(86,145)	(123,396)
Available for sale, fair value	22,751,894	21,490,292
Corporate Securities [Member] \| Fixed Maturity Securities [Member] \| Held For Investment Securities [Member]
Investment Holdings [Line Items]
Held for investment, amortized cost	76,479	76,432
Held for investment, gross unrealized gains	6,505	0
Held for investment, gross unrealized losses	0	(594)
Held for investment	82,984	75,838
Residential Mortgage Backed Securities [Member] \| Fixed Maturity Securities [Member] \| Available For Sale Securities [Member]
Investment Holdings [Line Items]
Available for sale, amortized cost	1,571,548	1,616,846
Available for sale, gross unrealized gains	159,281	136,704
Available for sale, gross unrealized losses	(1,514)	(2,205)
Available for sale, fair value	1,729,315	1,751,345
Commercial Mortgage Backed Securities [Member] \| Fixed Maturity Securities [Member] \| Available For Sale Securities [Member]
Investment Holdings [Line Items]
Available for sale, amortized cost	3,017,014	2,720,294
Available for sale, gross unrealized gains	149,269	90,649
Available for sale, gross unrealized losses	(454)	(3,323)
Available for sale, fair value	3,165,829	2,807,620
Other Asset Backed Securities [Member] \| Fixed Maturity Securities [Member] \| Available For Sale Securities [Member]
Investment Holdings [Line Items]
Available for sale, amortized cost	1,012,680	906,346
Available for sale, gross unrealized gains	54,766	48,022
Available for sale, gross unrealized losses	(8,137)	(7,885)
Available for sale, fair value	$ 1,059,309	$ 946,483

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

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