Investments (Details 3) (USD $) In Millions	Jun. 30, 2011	Dec. 31, 2010
Municipal Bonds
Investments
Fair Value	$ 4,814.4	$ 4,481.6
Amortized Cost	4,690.9	4,465.3
Municipal Bonds \| Texas
Investments
Fair Value	688.7	623.1
Amortized Cost	670.1	620.7
Municipal Bonds \| New York
Investments
Fair Value	631.8	547.6
Amortized Cost	616.0	545.1
Municipal Bonds \| California
Investments
Fair Value	364.6	403.4
Amortized Cost	352.9	401.0
Municipal Bonds \| Florida
Investments
Fair Value	333.9	303.9
Amortized Cost	323.8	300.5
Municipal Bonds \| Illinois
Investments
Fair Value	306.7	297.1
Amortized Cost	300.9	300.9
Municipal Bonds \| Washington
Investments
Fair Value	206.2	206.4
Amortized Cost	201.1	204.6
Municipal Bonds \| Massachusetts
Investments
Fair Value	203.0	183.9
Amortized Cost	198.5	185.8
Municipal Bonds \| Arizona
Investments
Fair Value	161.9	145.1
Amortized Cost	158.8	145.5
Municipal Bonds \| Michigan
Investments
Fair Value	121.1	132.6
Amortized Cost	117.6	131.4
Municipal Bonds \| Georgia
Investments
Fair Value	119.8	124.8
Amortized Cost	119.5	125.4
Municipal Bonds \| All other states
Investments
Fair Value	1,676.7	1,513.7
Amortized Cost	1,631.7	1,504.4
Pre-refunded Bond
Investments
Fair Value	369.8	478.3
State General Obligation
Investments
Fair Value	605.8	519.2
State General Obligation \| Texas
Investments
Fair Value	73.0	76.8
State General Obligation \| New York
Investments
Fair Value	11.7	11.4
State General Obligation \| California
Investments
Fair Value	17.8	17.2
State General Obligation \| Florida
Investments
Fair Value	44.5	45.2
State General Obligation \| Illinois
Investments
Fair Value	15.6	10.5
State General Obligation \| Washington
Investments
Fair Value	63.0	80.0
State General Obligation \| Massachusetts
Investments
Fair Value	39.8	38.0
State General Obligation \| Georgia
Investments
Fair Value	19.5	19.3
State General Obligation \| All other states
Investments
Fair Value	320.9	220.8
Local General Obligation
Investments
Fair Value	987.6	871.6
Local General Obligation \| Texas
Investments
Fair Value	331.6	294.4
Local General Obligation \| New York
Investments
Fair Value	54.5	40.0
Local General Obligation \| California
Investments
Fair Value	61.6	56.0
Local General Obligation \| Florida
Investments
Fair Value	53.6	45.1
Local General Obligation \| Illinois
Investments
Fair Value	90.8	91.0
Local General Obligation \| Washington
Investments
Fair Value	37.8	37.2
Local General Obligation \| Massachusetts
Investments
Fair Value	8.9	8.6
Local General Obligation \| Arizona
Investments
Fair Value	7.2	0.6
Local General Obligation \| Michigan
Investments
Fair Value	39.7	39.5
Local General Obligation \| Georgia
Investments
Fair Value	38.7	38.4
Local General Obligation \| All other states
Investments
Fair Value	263.2	220.8
Revenue bonds
Investments
Fair Value	3,221.0	3,090.8
Amortized Cost	3,145.1	3,085.9
Revenue bonds \| Texas
Investments
Fair Value	284.1	251.9
Revenue bonds \| New York
Investments
Fair Value	565.6	496.2
Revenue bonds \| California
Investments
Fair Value	285.2	330.2
Revenue bonds \| Florida
Investments
Fair Value	235.8	213.6
Revenue bonds \| Illinois
Investments
Fair Value	200.3	195.6
Revenue bonds \| Washington
Investments
Fair Value	105.4	89.2
Revenue bonds \| Massachusetts
Investments
Fair Value	154.3	137.3
Revenue bonds \| Arizona
Investments
Fair Value	154.7	144.5
Revenue bonds \| Michigan
Investments
Fair Value	81.4	93.1
Revenue bonds \| Georgia
Investments
Fair Value	61.6	67.1
Revenue bonds \| All other states
Investments
Fair Value	1,092.6	1,072.1
Revenue bonds \| Transportation
Investments
Fair Value	760.7	725.5
Amortized Cost	742.2	718.9
Revenue bonds \| Municipal utilities
Investments
Fair Value	502.9	457.8
Amortized Cost	487.8	456.8
Revenue bonds \| Water and sewer
Investments
Fair Value	486.8	470.6
Amortized Cost	478.6	471.3
Revenue bonds \| Higher Education
Investments
Fair Value	299.3	298.2
Amortized Cost	293.1	302.1
Revenue bonds \| Tax Backed
Investments
Fair Value	634.4	609.0
Amortized Cost	616.6	607.2
Revenue bonds \| Healthcare
Investments
Fair Value	241.1	207.3
Amortized Cost	234.9	206.5
Revenue bonds \| All other revenue sources
Investments
Fair Value	295.8	322.4
Amortized Cost	$ 291.9	$ 323.1

- Definition

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.

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