Summary of Investments (Details) (USD $) In Thousands, unless otherwise specified	9 Months Ended	12 Months Ended
Sep. 30, 2013	Dec. 31, 2012
Securities available-for-sale
Amortized cost	$ 15,509,373	$ 16,756,216
Estimated fair value and carrying value	16,249,035	18,188,344
Fixed maturities
Securities available-for-sale
Amortized cost	15,509,373	16,756,216
Gross unrealized gains	1,024,836	1,604,876
Gross unrealized losses	285,174	172,748
Estimated fair value and carrying value	16,249,035	18,188,344
OTTI (gain) loss included in AOCI	(86,026)	(68,826)
U.S. government direct obligations and U.S. agencies
Securities available-for-sale
Amortized cost	880,358	2,735,917
Gross unrealized gains	54,286	101,568
Gross unrealized losses	7,739	3,411
Estimated fair value and carrying value	926,905	2,834,074
Obligations of U.S. states and their subdivisions
Securities available-for-sale
Amortized cost	1,768,156	1,676,289
Gross unrealized gains	217,037	342,445
Gross unrealized losses	13,535	229
Estimated fair value and carrying value	1,971,658	2,018,505
Foreign government securities
Securities available-for-sale
Amortized cost	2,657
Gross unrealized losses	12
Estimated fair value and carrying value	2,645
Corporate debt securities
Securities available-for-sale
Amortized cost	10,290,656	9,511,411
Gross unrealized gains	600,229	974,231
Gross unrealized losses	224,131	111,551
Estimated fair value and carrying value	10,666,754	10,374,091
OTTI (gain) loss included in AOCI	(2,343)	(2,293)
Perpetual debt investments
Securities available-for-sale
Amortized cost	172,054	226,069
Estimated fair value and carrying value	134,641	153,100
Asset-backed securities
Securities available-for-sale
Amortized cost	1,671,565	1,795,122
Gross unrealized gains	119,243	120,471
Gross unrealized losses	29,605	54,454
Estimated fair value and carrying value	1,761,203	1,861,139
OTTI (gain) loss included in AOCI	(83,505)	(66,293)
Residential mortgage-backed securities
Securities available-for-sale
Amortized cost	311,397	407,715
Gross unrealized gains	10,483	17,900
Gross unrealized losses	2,634	30
Estimated fair value and carrying value	319,246	425,585
OTTI (gain) loss included in AOCI	(178)	(240)
Commercial mortgage-backed securities
Securities available-for-sale
Amortized cost	571,740	616,011
Gross unrealized gains	23,544	48,247
Gross unrealized losses	6,894	1,303
Estimated fair value and carrying value	588,390	662,955
Collateralized debt obligations
Securities available-for-sale
Amortized cost	12,844	13,751
Gross unrealized gains	14	14
Gross unrealized losses	624	1,770
Estimated fair value and carrying value	$ 12,234	$ 11,995

Amount of debt securities categorized neither as held-to-maturity nor trading.

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 115

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Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

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Reference 3: http://www.xbrl.org/2003/role/presentationRef

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Reference 4: http://www.xbrl.org/2003/role/presentationRef

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Reference 5: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 115

-Paragraph 13

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Reference 6: http://www.xbrl.org/2003/role/presentationRef

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-Number FAS115-1/124-1

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Reference 9: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 320

-SubTopic 10

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Reference 10: http://www.xbrl.org/2003/role/presentationRef

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+ Details

- Definition

This item represents the gross unrealized gains for securities, at a point in time, which are categorized neither as held-to-maturity nor trading securities.

+ References+ Details

- Definition

Before tax amount of other than temporary impairment (OTTI) on a debt security, categorized as available-for-sale, recognized in other comprehensive income (loss), attributable to the parent entity.

+ References+ Details