Summary of Investments (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
Dec. 31, 2012	Dec. 31, 2011
Securities available-for-sale
Amortized cost	$ 16,756,216	$ 15,586,970
Estimated fair value and carrying value	18,188,344	16,589,783
Fixed maturities
Securities available-for-sale
Amortized cost	16,756,216	15,586,970
Gross unrealized gains	1,604,876	1,297,015
Gross unrealized losses	172,748	294,202
Estimated fair value and carrying value	18,188,344	16,589,783
OTTI (gain) loss included in AOCI	(68,826)	(18,756)
U.S. government direct obligations and U.S. agencies
Securities available-for-sale
Amortized cost	2,735,917	2,209,420
Gross unrealized gains	101,568	107,363
Gross unrealized losses	3,411	1,112
Estimated fair value and carrying value	2,834,074	2,315,671
Obligations of U.S. states and their subdivisions
Securities available-for-sale
Amortized cost	1,676,289	1,773,687
Gross unrealized gains	342,445	297,488
Gross unrealized losses	229	5
Estimated fair value and carrying value	2,018,505	2,071,170
Corporate debt securities
Securities available-for-sale
Amortized cost	9,511,411	8,287,960
Gross unrealized gains	974,231	762,045
Gross unrealized losses	111,551	154,259
Estimated fair value and carrying value	10,374,091	8,895,746
OTTI (gain) loss included in AOCI	(2,293)	3,672
Perpetual debt investments
Securities available-for-sale
Amortized cost	226,069	253,023
Estimated fair value and carrying value	153,100	166,284
Asset-backed securities
Securities available-for-sale
Amortized cost	1,795,122	2,006,544
Gross unrealized gains	120,471	70,117
Gross unrealized losses	54,454	125,217
Estimated fair value and carrying value	1,861,139	1,951,444
OTTI (gain) loss included in AOCI	(66,293)	(23,837)
Residential mortgage-backed securities
Securities available-for-sale
Amortized cost	407,715	578,046
Gross unrealized gains	17,900	17,461
Gross unrealized losses	30	3,965
Estimated fair value and carrying value	425,585	591,542
OTTI (gain) loss included in AOCI	(240)	1,409
Commercial mortgage-backed securities
Securities available-for-sale
Amortized cost	616,011	712,831
Gross unrealized gains	48,247	42,538
Gross unrealized losses	1,303	7,572
Estimated fair value and carrying value	662,955	747,797
Collateralized debt obligations
Securities available-for-sale
Amortized cost	13,751	18,482
Gross unrealized gains	14	3
Gross unrealized losses	1,770	2,072
Estimated fair value and carrying value	$ 11,995	$ 16,413

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

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

-Publisher FASB

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-Number FAS115-1/124-1

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

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 115

-Paragraph 12

-Subparagraph b

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

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 115

-Paragraph 137

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

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 115

-Paragraph 16

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 942

-SubTopic 320

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-Name Accounting Standards Codification

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-Publisher FASB

-Name Accounting Standards Codification

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-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 133

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

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-Number 115

-Paragraph 13

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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