Fair Value (Financial Assets Measured at Fair Value on a Recurring Basis) (Details) (USD $) In Thousands	Jun. 30, 2011	Dec. 31, 2010
Cash equivalents	$ 1,470,957	$ 1,606,592
Total debt securities	9,202,656	8,372,439
Securities lending invested collateral	29,737	49,636
Total invested assets	10,703,350	10,028,667
U.S. Treasury and Agency Obligations [Member] \| Fair Value, Inputs, Level 1 [Member] \| Debt Securities [Member] \| U.S. Treasury and Government [Member]
Total debt securities	0	0
Mortgage-Backed Securities [Member] \| Fair Value, Inputs, Level 1 [Member] \| Debt Securities [Member] \| U.S. 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	1,470,957	1,606,592
Total debt securities	0	0
Securities lending invested collateral	29,737	24,639
Total invested assets	1,500,694	1,631,231
U.S. Treasury and Agency Obligations [Member] \| Fair Value, Inputs, Level 2 [Member] \| Debt Securities [Member] \| U.S. Treasury and Government [Member]
Total debt securities	822,787	711,613
Mortgage-Backed Securities [Member] \| Fair Value, Inputs, Level 2 [Member] \| Debt Securities [Member] \| U.S. Treasury and Government [Member]
Total debt securities	1,824,036	1,663,179
Tax-Exempt Municipal Securities [Member] \| Fair Value, Inputs, Level 2 [Member] \| Debt Securities [Member]
Total debt securities	2,468,830	2,381,528
Residential Mortgage-Backed Securities [Member] \| Fair Value, Inputs, Level 2 [Member] \| Debt Securities [Member]
Total debt securities	49,749	55,887
Commercial Mortgage-Backed Securities [Member] \| Fair Value, Inputs, Level 2 [Member] \| Debt Securities [Member]
Total debt securities	414,963	321,031
Asset-Backed Securities [Member] \| Fair Value, Inputs, Level 2 [Member] \| Debt Securities [Member]
Total debt securities	116,286	148,545
Corporate Debt Securities [Member] \| Fair Value, Inputs, Level 2 [Member] \| Debt Securities [Member]
Total debt securities	3,454,422	3,025,097
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	9,151,073	8,306,880
Securities lending invested collateral	0	24,997
Total invested assets	9,151,073	8,331,877
U.S. Treasury and Agency Obligations [Member] \| Fair Value, Inputs, Level 3 [Member] \| Debt Securities [Member] \| U.S. Treasury and Government [Member]
Total debt securities	0	0
Mortgage-Backed Securities [Member] \| Fair Value, Inputs, Level 3 [Member] \| Debt Securities [Member] \| U.S. 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	21,053	51,806
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	1,099	1,206
Corporate Debt Securities [Member] \| Fair Value, Inputs, Level 3 [Member] \| Debt Securities [Member]
Total debt securities	24,098	7,214
Redeemable Preferred Stock [Member] \| Fair Value, Inputs, Level 3 [Member] \| Debt Securities [Member]
Total debt securities	5,333	5,333
Fair Value, Inputs, Level 3 [Member]
Cash equivalents	0	0
Total debt securities	51,583	65,559
Securities lending invested collateral	0	0
Total invested assets	51,583	65,559
U.S. Treasury and Agency Obligations [Member] \| Debt Securities [Member] \| U.S. Treasury and Government [Member]
Total debt securities	822,787	711,613
Mortgage-Backed Securities [Member] \| Debt Securities [Member] \| U.S. Treasury and Government [Member]
Total debt securities	1,824,036	1,663,179
Tax-Exempt Municipal Securities [Member] \| Debt Securities [Member]
Total debt securities	2,489,883	2,433,334
Residential Mortgage-Backed Securities [Member] \| Debt Securities [Member]
Total debt securities	49,749	55,887
Commercial Mortgage-Backed Securities [Member] \| Debt Securities [Member]
Total debt securities	414,963	321,031
Asset-Backed Securities [Member] \| Debt Securities [Member]
Total debt securities	117,385	149,751
Corporate Debt Securities [Member] \| Debt Securities [Member]
Total debt securities	3,478,520	3,032,311
Redeemable Preferred Stock [Member] \| Debt Securities [Member]
Total debt securities	$ 5,333	$ 5,333

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.

+ References+ Details

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.

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- 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