Investments - Amortized Cost and Estimated Fair Value of investments (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Schedule of Available-for-sale Securities [Line Items]
Investments in Fixed Maturities, Amortized Cost	$ 4,283.8
Investments in Fixed Maturities, Fair Value	4,860.2	4,773.4
Investments in Fixed Maturities [Member]
Schedule of Available-for-sale Securities [Line Items]
Investments in Fixed Maturities, Amortized Cost	4,283.8	4,266.1
Gross Unrealized Gains	584.5	518.4
Gross Unrealized Losses	(8.1)	(11.1)
Investments in Fixed Maturities, Fair Value	4,860.2	4,773.4
US Government and Government Agencies and Authorities [Member]
Schedule of Available-for-sale Securities [Line Items]
Investments in Fixed Maturities, Amortized Cost	384.0	439.4
Gross Unrealized Gains	45.1	52.3
Gross Unrealized Losses	(0.2)	0
Investments in Fixed Maturities, Fair Value	428.9	491.7
States and Political Subdivisions [Member]
Schedule of Available-for-sale Securities [Line Items]
Investments in Fixed Maturities, Amortized Cost	1,251.0	1,705.0
Gross Unrealized Gains	150.5	148.4
Gross Unrealized Losses	(0.1)	(0.8)
Investments in Fixed Maturities, Fair Value	1,401.4	1,852.6
Corporate Bonds and Notes [Member]
Schedule of Available-for-sale Securities [Line Items]
Investments in Fixed Maturities, Amortized Cost	2,615.5	2,040.1
Gross Unrealized Gains	385.4	311.6
Gross Unrealized Losses	(7.5)	(9.4)
Investments in Fixed Maturities, Fair Value	2,993.4	2,342.3
Redeemable Preferred Stocks [Member]
Schedule of Available-for-sale Securities [Line Items]
Investments in Fixed Maturities, Amortized Cost	30.1	76.7
Gross Unrealized Gains	2.5	5.1
Gross Unrealized Losses	0	(0.1)
Investments in Fixed Maturities, Fair Value	32.6	81.7
Mortgage and Asset Backed [Member]
Schedule of Available-for-sale Securities [Line Items]
Investments in Fixed Maturities, Amortized Cost	3.2	4.9
Gross Unrealized Gains	1.0	1.0
Gross Unrealized Losses	(0.3)	(0.8)
Investments in Fixed Maturities, Fair Value	3.9	5.1
Collateralized Debt Obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Investments in Fixed Maturities, Fair Value	2.3	2.9
Mortgage-backed Securities, Issued by Private Enterprises [Member]
Schedule of Available-for-sale Securities [Line Items]
Investments in Fixed Maturities, Fair Value	1.3	1.7
Other Asset Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Investments in Fixed Maturities, Fair Value	0.3	0.4
Commercial Mortgage Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Investments in Fixed Maturities, Fair Value		$ 0.1

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

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

-Publisher FASB

-Name FASB Staff Position (FSP)

-Number FAS115-1/124-1

-Paragraph 15C

-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 Statement of Financial Accounting Standard (FAS)

-Number 115

-Paragraph 12

-Subparagraph b

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

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 115

-Paragraph 137

-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 Statement of Financial Accounting Standard (FAS)

-Number 115

-Paragraph 16

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 942

-SubTopic 320

-Section 50

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

-Publisher FASB

-Name Accounting Standards Codification

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Reference 7: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

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Reference 8: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 133

-Paragraph 22

-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 9: 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 10: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name FASB Staff Position (FSP)

-Number FAS115-1/124-1

-Paragraph 15D

-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

- 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