Fair Value (Financial Assets Measured At Fair Value On A Recurring Basis) (Details) (USD $) In Thousands	Dec. 31, 2010	Dec. 31, 2009
Cash equivalents	$ 1,606,592	$ 1,507,490
Total debt securities	8,372,439	7,497,150
Securities lending invested collateral	49,636	119,586
Total invested assets	10,028,667	9,124,226
US Treasury And Agency Obligations [Member] \| Fair Value, Inputs, Level 1 [Member] \| Debt Securities [Member] \| US Treasury And Government [Member]
Total debt securities	0	0
Mortgage-Backed Securities [Member] \| Fair Value, Inputs, Level 1 [Member] \| Debt Securities [Member] \| US Treasury And Government [Member]
Total debt securities	0	0
Tax-Exempt Municipal Securities [Member] \| Fair Value, Inputs, Level 1 [Member] \| Debt Securities [Member]
Total debt securities	0	0
Residential Mortgage Backed Securities [Member] \| Fair Value, Inputs, Level 1 [Member] \| Debt Securities [Member]
Total debt securities	0	0
Commercial Mortgage Backed Securities [Member] \| Fair Value, Inputs, Level 1 [Member] \| Debt Securities [Member]
Total debt securities	0	0
Asset-Backed Securities [Member] \| Fair Value, Inputs, Level 1 [Member] \| Debt Securities [Member]
Total debt securities	0	0
Corporate Debt Securities [Member] \| Fair Value, Inputs, Level 1 [Member] \| Debt Securities [Member]
Total debt securities	0	0
Redeemable Preferred Stock [Member] \| Fair Value, Inputs, Level 1 [Member] \| Debt Securities [Member]
Total debt securities	0	0
Fair Value, Inputs, Level 1 [Member]
Cash equivalents	1,606,592	1,507,490
Total debt securities	0	0
Securities lending invested collateral	24,639	53,569
Total invested assets	1,631,231	1,561,059
US Treasury And Agency Obligations [Member] \| Fair Value, Inputs, Level 2 [Member] \| Debt Securities [Member] \| US Treasury And Government [Member]
Total debt securities	711,613	1,009,352
Mortgage-Backed Securities [Member] \| Fair Value, Inputs, Level 2 [Member] \| Debt Securities [Member] \| US Treasury And Government [Member]
Total debt securities	1,663,179	1,688,663
Tax-Exempt Municipal Securities [Member] \| Fair Value, Inputs, Level 2 [Member] \| Debt Securities [Member]
Total debt securities	2,381,528	2,155,227
Residential Mortgage Backed Securities [Member] \| Fair Value, Inputs, Level 2 [Member] \| Debt Securities [Member]
Total debt securities	55,887	95,412
Commercial Mortgage Backed Securities [Member] \| Fair Value, Inputs, Level 2 [Member] \| Debt Securities [Member]
Total debt securities	321,031	279,626
Asset-Backed Securities [Member] \| Fair Value, Inputs, Level 2 [Member] \| Debt Securities [Member]
Total debt securities	148,545	105,060
Corporate Debt Securities [Member] \| Fair Value, Inputs, Level 2 [Member] \| Debt Securities [Member]
Total debt securities	3,025,097	2,071,087
Redeemable Preferred Stock [Member] \| Fair Value, Inputs, Level 2 [Member] \| Debt Securities [Member]
Total debt securities	0	0
Fair Value, Inputs, Level 2 [Member]
Cash equivalents	0	0
Total debt securities	8,306,880	7,404,427
Securities lending invested collateral	24,997	66,017
Total invested assets	8,331,877	7,470,444
US Treasury And Agency Obligations [Member] \| Fair Value, Inputs, Level 3 [Member] \| Debt Securities [Member] \| US Treasury And Government [Member]
Total debt securities	0	0
Mortgage-Backed Securities [Member] \| Fair Value, Inputs, Level 3 [Member] \| Debt Securities [Member] \| US Treasury And Government [Member]
Total debt securities	0	0
Tax-Exempt Municipal Securities [Member] \| Fair Value, Inputs, Level 3 [Member] \| Debt Securities [Member]
Total debt securities	51,806	68,814
Residential Mortgage Backed Securities [Member] \| Fair Value, Inputs, Level 3 [Member] \| Debt Securities [Member]
Total debt securities	0	0
Commercial Mortgage Backed Securities [Member] \| Fair Value, Inputs, Level 3 [Member] \| Debt Securities [Member]
Total debt securities	0	0
Asset-Backed Securities [Member] \| Fair Value, Inputs, Level 3 [Member] \| Debt Securities [Member]
Total debt securities	1,206	2,128
Corporate Debt Securities [Member] \| Fair Value, Inputs, Level 3 [Member] \| Debt Securities [Member]
Total debt securities	7,214	8,481
Redeemable Preferred Stock [Member] \| Fair Value, Inputs, Level 3 [Member] \| Debt Securities [Member]
Total debt securities	5,333	13,300
Fair Value, Inputs, Level 3 [Member]
Cash equivalents	0	0
Total debt securities	65,559	92,723
Securities lending invested collateral	0	0
Total invested assets	65,559	92,723
US Treasury And Agency Obligations [Member] \| Debt Securities [Member] \| US Treasury And Government [Member]
Total debt securities	711,613	1,009,352
Mortgage-Backed Securities [Member] \| Debt Securities [Member] \| US Treasury And Government [Member]
Total debt securities	1,663,179	1,688,663
Tax-Exempt Municipal Securities [Member] \| Debt Securities [Member]
Total debt securities	2,433,334	2,224,041
Residential Mortgage Backed Securities [Member] \| Debt Securities [Member]
Total debt securities	55,887	95,412
Commercial Mortgage Backed Securities [Member] \| Debt Securities [Member]
Total debt securities	321,031	279,626
Asset-Backed Securities [Member] \| Debt Securities [Member]
Total debt securities	149,751	107,188
Corporate Debt Securities [Member] \| Debt Securities [Member]
Total debt securities	3,032,311	2,079,568
Redeemable Preferred Stock [Member] \| Debt Securities [Member]
Total debt securities	$ 5,333	$ 13,300

This element represents the portion of the balance sheet assertion valued at fair value by the entity whether such amount is presented as a separate caption or as a parenthetical disclosure. Additionally, this element may be used in connection with the fair value disclosures required in the footnote disclosures to the financial statements. The element may be used in both the balance sheet and disclosure in the same submission. This item represents Available-for-sale Securities which consist of all investments in certain debt and equity securities neither classified as trading or held-to-maturity securities. A debt security represents a creditor relationship with an enterprise. Debt securities include, among other items, US Treasury securities, US government securities, municipal securities, corporate bonds, convertible debt, commercial paper, and all securitized debt instruments. An equity security represents an ownership interest in an enterprise or the right to acquire or dispose of an ownership interest in an enterprise at fixed or determinable prices. Equity securities include, among other things, common stock, certain preferred stock, warrant rights, call options, and put options, but do not include convertible debt. An entity may opt to provide the reader with additional narrative text to better understand the nature of investments in debt and equity securities which are categorized as Available-for-sale.

This element represents the portion of the balance sheet assertion valued at fair value by the entity whether such amount is presented as a separate caption or as a parenthetical disclosure. Additionally, this element may be used in connection with the fair value disclosures required in the footnote disclosures to the financial statements. The element may be used in both the balance sheet and disclosure in the same submission. This item includes currency on hand as well as demand deposits with banks or financial institutions. It also includes other kinds of accounts that have the general characteristics of demand deposits in that the Company may deposit additional funds at any time and also effectively may withdraw funds at any time without prior notice or penalty. Cash equivalents, excluding items classified as marketable securities, include short-term, highly liquid investments that are both readily convertible to known amounts of cash, and so near their maturity that they present minimal risk of changes in value because of changes in interest rates. Generally, only investments with original maturities of three months or less qualify under that definition. Original maturity means original maturity to the entity holding the investment. For example, both a three-month US Treasury bill and a three-year Treasury note purchased three months from maturity qualify as cash equivalents. However, a Treasury note purchased three years ago does not become a cash equivalent when its remaining maturity is three months.

+ References+ Details

- Definition

The carrying value as of the balance sheet date of the assets received as collateral against securities loaned to other broker-dealers. Borrowers of securities generally are required to provide collateral to the lenders of securities, commonly cash but sometimes other securities or standby letters of credit, with a value slightly higher than that of the securities borrowed. In instances where the entity is permitted to sell or re-pledge these securities, the entity reports the fair value of the collateral received and the related obligation to return the collateral as a liability.

+ References

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

-Publisher AICPA

-Name Audit and Accounting Guide (AAG)

-Number AAG-BRD

-Chapter 4

-Paragraph 49

-IssueDate 2006-05-01

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 860

-SubTopic 10

-Section 05

-Paragraph 17

-URI http://asc.fasb.org/extlink&oid=6901665&loc=d3e93645-111703

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

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 140

-Paragraph 17

-Subparagraph a(3)

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 860

-SubTopic 30

-Section 50

-Paragraph 1

-Subparagraph (c)(1)

-URI http://asc.fasb.org/extlink&oid=6862952&loc=d3e113969-111729

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

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 140

-Paragraph 15

-Subparagraph a, b, c, d

-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)

-Number 140

-Paragraph 91

-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.

+ Details