Summary of Investments (Details) (USD $) In Thousands, unless otherwise specified	6 Months Ended	12 Months Ended
Jun. 30, 2013	Dec. 31, 2012
Securities available-for-sale
Amortized cost	$ 15,509,767	$ 16,756,216
Estimated fair value and carrying value	16,281,559	18,188,344
Fixed maturities
Securities available-for-sale
Amortized cost	15,509,767	16,756,216
Gross unrealized gains	1,068,414	1,604,876
Gross unrealized losses	296,622	172,748
Estimated fair value and carrying value	16,281,559	18,188,344
OTTI (gain) loss included in AOCI	(81,321)	(68,826)
U.S. government direct obligations and U.S. agencies
Securities available-for-sale
Amortized cost	923,046	2,735,917
Gross unrealized gains	56,313	101,568
Gross unrealized losses	6,875	3,411
Estimated fair value and carrying value	972,484	2,834,074
Obligations of U.S. states and their subdivisions
Securities available-for-sale
Amortized cost	1,731,629	1,676,289
Gross unrealized gains	235,248	342,445
Gross unrealized losses	7,784	229
Estimated fair value and carrying value	1,959,093	2,018,505
Foreign government securities
Securities available-for-sale
Amortized cost	2,698
Gross unrealized losses	25
Estimated fair value and carrying value	2,673
Corporate debt securities
Securities available-for-sale
Amortized cost	10,164,568	9,511,411
Gross unrealized gains	618,701	974,231
Gross unrealized losses	240,214	111,551
Estimated fair value and carrying value	10,543,055	10,374,091
OTTI (gain) loss included in AOCI	(2,455)	(2,293)
Perpetual debt investments
Securities available-for-sale
Amortized cost	172,054	226,069
Estimated fair value and carrying value	135,573	153,100
Asset-backed securities
Securities available-for-sale
Amortized cost	1,743,165	1,795,122
Gross unrealized gains	120,098	120,471
Gross unrealized losses	32,660	54,454
Estimated fair value and carrying value	1,830,603	1,861,139
OTTI (gain) loss included in AOCI	(78,652)	(66,293)
Residential mortgage-backed securities
Securities available-for-sale
Amortized cost	338,222	407,715
Gross unrealized gains	11,948	17,900
Gross unrealized losses	2,490	30
Estimated fair value and carrying value	347,680	425,585
OTTI (gain) loss included in AOCI	(214)	(240)
Commercial mortgage-backed securities
Securities available-for-sale
Amortized cost	593,273	616,011
Gross unrealized gains	26,092	48,247
Gross unrealized losses	5,976	1,303
Estimated fair value and carrying value	613,389	662,955
Collateralized debt obligations
Securities available-for-sale
Amortized cost	13,166	13,751
Gross unrealized gains	14	14
Gross unrealized losses	598	1,770
Estimated fair value and carrying value	$ 12,582	$ 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

-Name Accounting Standards Codification

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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

-Publisher FASB

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-Number 115

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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

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

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

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

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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

-Section 45

-Paragraph 1

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

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-Section 50

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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