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(Rollforward of Fair Value Measurements by Level) (Details) (USD $) | 6 Months Ended | 12 Months Ended |
--- | --- | --- | ---
Jun. 30, 2013 | Jun. 30, 2012 | Dec. 31, 2012 | Dec. 31, 2011
Investment securities |  |  |  |  |  |  |
Fixed maturity investments reclassified to assets held for sale |  |  |  |  | $ (338,100,000) |  |
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Transfers Into Level 3 | 8.0 |  | 21.0 |  |  |  |
Level 1 Inputs |  |  |  |  |  |  |
Investment securities |  |  |  |  |  |  |
Fair value investments | 1,355,100,000 | [1] | 1,033,100,000 | [2] | 1,033,100,000 | [2] |
Total realized and unrealized gains (losses) | 89,900,000 | [1],[3] | 3,000,000 | [2],[4] |  |  |
Foreign currency gains (losses) through OCI | (5,600,000) | [1] | 300,000 | [2] |  |  |
Amortization/Accretion | (600,000) | [1] | (200,000) | [2] |  |  |
Purchases | 549,000,000 | [1] | 596,300,000 | [2] |  |  |
Sales | (583,400,000) | [1] | (481,400,000) | [2] |  |  |
Net change in investments related to purchases and sales of consolidated affiliates | 14,500,000 | [1] |  |  |  |  |
Transfers in | 1,700,000 | [1] | 500,000 | [2] |  |  |
Transfers out | 0 | [1] | (100,000) | [2] |  |  |
Fair value investments | 1,420,600,000 | [1] | 1,151,500,000 | [2] |  |  |
Level 2 Inputs |  |  |  |  |  |  |
Investment securities |  |  |  |  |  |  |
Fair value investments | 5,206,100,000 |  | 6,088,200,000 |  | 6,088,200,000 |  |
Total realized and unrealized gains (losses) | (55,300,000) | [3] | 44,800,000 | [4] |  |  |
Foreign currency gains (losses) through OCI | (44,200,000) |  | (10,200,000) |  |  |  |
Amortization/Accretion | (27,000,000) |  | (23,300,000) |  |  |  |
Purchases | 2,345,500,000 |  | 2,757,600,000 |  |  |  |
Sales | (2,713,900,000) |  | (3,871,300,000) |  |  |  |
Net change in investments related to purchases and sales of consolidated affiliates | 2,700,000 |  |  |  |  |  |
Transfers in | 8,000,000 |  | 21,100,000 |  |  |  |
Transfers out | (700,000) |  | (500,000) |  |  |  |
Fair value investments | 4,721,200,000 |  | 5,006,400,000 |  |  |  |
Fair Value |  |  |  |  |  |  |
Investment securities |  |  |  |  |  |  |
Fair value investments | 6,950,700,000 | [5],[6] | 7,500,800,000 | [7],[8] | 7,500,800,000 | [7],[8] |
Total realized and unrealized gains (losses) | 49,300,000 | [3] | 52,900,000 | [4] |  |  |
Foreign currency gains (losses) through OCI | (51,300,000) |  | (10,500,000) |  |  |  |
Amortization/Accretion | (27,600,000) |  | (24,300,000) |  |  |  |
Purchases | 2,963,200,000 |  | 3,515,000,000 |  |  |  |
Sales | (3,336,000,000) |  | (4,462,000,000) |  |  |  |
Net change in investments related to purchases and sales of consolidated affiliates | 17,200,000 |  |  |  |  |  |
Transfers in | 9,700,000 |  | 21,600,000 |  |  |  |
Transfers out | (9,700,000) |  | (21,600,000) |  |  |  |
Fair value investments | 6,565,500,000 | [5],[6] | 6,571,900,000 | [7],[8] |  |  |
Fixed maturity investments. | Level 3 Inputs |  |  |  |  |  |  |
Investment securities |  |  |  |  |  |  |
Fair value investments | 92,900,000 |  | 78,900,000 |  | 78,900,000 |  |
Total realized and unrealized gains (losses) | (300,000) | [3] | 7,400,000 | [4] |  |  |
Foreign currency gains (losses) through OCI | 0 |  | 700,000 |  |  |  |
Amortization/Accretion | 0 |  | (800,000) |  |  |  |
Purchases | 32,300,000 |  | 138,800,000 |  |  |  |
Sales | 0 |  | (84,300,000) |  |  |  |
Net change in investments related to purchases and sales of consolidated affiliates | 0 |  |  |  |  |  |
Transfers in | 0 |  | 0 |  |  |  |
Transfers out | (8,000,000) |  | (21,000,000) |  |  |  |
Fair value investments | 116,900,000 |  | 119,700,000 |  |  |  |
Common equity securities | Level 3 Inputs |  |  |  |  |  |  |
Investment securities |  |  |  |  |  |  |
Fair value investments | 37,300,000 |  | 32,300,000 |  | 32,300,000 |  |
Total realized and unrealized gains (losses) | 800,000 | [3] | 1,000,000 | [4] |  |  |
Foreign currency gains (losses) through OCI | 0 |  | 0 |  |  |  |
Amortization/Accretion | 0 |  | 0 |  |  |  |
Purchases | 500,000 |  | 2,500,000 |  |  |  |
Sales | 0 |  | 0 |  |  |  |
Net change in investments related to purchases and sales of consolidated affiliates | 0 |  |  |  |  |  |
Transfers in | 0 |  | 0 |  |  |  |
Transfers out | (1,000,000) |  | 0 |  |  |  |
Fair value investments | 37,600,000 |  | 35,800,000 |  |  |  |
Convertible Debt Securities | Level 3 Inputs |  |  |  |  |  |  |
Investment securities |  |  |  |  |  |  |
Fair value investments | 0 |  | 0 |  | 0 |  |
Total realized and unrealized gains (losses) | 0 | [3] | 0 | [4] |  |  |
Foreign currency gains (losses) through OCI | 0 |  | 0 |  |  |  |
Amortization/Accretion | 0 |  | 0 |  |  |  |
Purchases | 0 |  | 0 |  |  |  |
Sales | 0 |  | 0 |  |  |  |
Net change in investments related to purchases and sales of consolidated affiliates | 0 |  |  |  |  |  |
Transfers in | 0 |  | 0 |  |  |  |
Transfers out | 0 |  | 0 |  |  |  |
Fair value investments | 0 |  | 0 |  |  |  |
Other long-term investments | Level 3 Inputs |  |  |  |  |  |  |
Investment securities |  |  |  |  |  |  |
Fair value investments | 259,300,000 |  | 268,300,000 | [7],[8] | 268,300,000 | [7],[8] |
Total realized and unrealized gains (losses) | 14,200,000 | [3] | (3,300,000) | [4] |  |  |
Foreign currency gains (losses) through OCI | (1,500,000) |  | (1,300,000) |  |  |  |
Amortization/Accretion | 0 |  |  |  |  |  |
Purchases | 35,900,000 |  | 19,800,000 |  |  |  |
Sales | (38,700,000) |  | (25,000,000) |  |  |  |
Net change in investments related to purchases and sales of consolidated affiliates | 0 |  |  |  |  |  |
Transfers in | 0 |  | 0 |  |  |  |
Transfers out | 0 |  | 0 |  |  |  |
Fair value investments | 269,200,000 |  | 258,500,000 | [8] |  |  |
Fair value measured on a recurring basis | Level 1 Inputs |  |  |  |  |  |  |
Investment securities |  |  |  |  |  |  |
Fair value investments | 2,002,700,000 |  |  |  | 1,985,700,000 |  |
Fair value measured on a recurring basis | Level 2 Inputs |  |  |  |  |  |  |
Investment securities |  |  |  |  |  |  |
Fair value investments | 4,745,900,000 |  |  |  | 5,206,100,000 |  |
Fair value measured on a recurring basis | Level 3 Inputs |  |  |  |  |  |  |
Investment securities |  |  |  |  |  |  |
Fair value investments | 423,700,000 |  |  |  | 389,500,000 |  |
Fair value measured on a recurring basis | Fair Value |  |  |  |  |  |  |
Investment securities |  |  |  |  |  |  |
Fair value investments | 7,172,300,000 |  |  |  | 7,581,300,000 |  |
Fair value measured on a recurring basis | Fixed maturity investments. | Level 1 Inputs |  |  |  |  |  |  |
Investment securities |  |  |  |  |  |  |
Fair value investments | 492,200,000 | [5] |  |  | 422,200,000 | [9] |
Fair value measured on a recurring basis | Fixed maturity investments. | Level 2 Inputs |  |  |  |  |  |  |
Investment securities |  |  |  |  |  |  |
Fair value investments | 4,583,600,000 | [5] |  |  | 5,019,200,000 | [9] |
Fair value measured on a recurring basis | Fixed maturity investments. | Level 3 Inputs |  |  |  |  |  |  |
Investment securities |  |  |  |  |  |  |
Fair value investments | 116,900,000 | [5] |  |  | 92,900,000 | [9] |
Fair value measured on a recurring basis | Fixed maturity investments. | Fair Value |  |  |  |  |  |  |
Investment securities |  |  |  |  |  |  |
Fair value investments | 5,192,700,000 | [5] |  |  | 5,534,300,000 | [9] |
Fair value measured on a recurring basis | Convertible Debt Securities | Level 1 Inputs |  |  |  |  |  |  |
Investment securities |  |  |  |  |  |  |
Fair value investments | 0 |  |  |  | 0 |  |
Fair value measured on a recurring basis | Convertible Debt Securities | Level 2 Inputs |  |  |  |  |  |  |
Investment securities |  |  |  |  |  |  |
Fair value investments | 74,100,000 |  |  |  | 127,400,000 |  |
Fair value measured on a recurring basis | Convertible Debt Securities | Level 3 Inputs |  |  |  |  |  |  |
Investment securities |  |  |  |  |  |  |
Fair value investments | 0 |  |  |  | 0 |  |
Fair value measured on a recurring basis | Convertible Debt Securities | Fair Value |  |  |  |  |  |  |
Investment securities |  |  |  |  |  |  |
Fair value investments | 74,100,000 |  |  |  | 127,400,000 |  |
Fair value measured on a recurring basis | Other long-term investments | Level 1 Inputs |  |  |  |  |  |  |
Investment securities |  |  |  |  |  |  |
Fair value investments | 0 | [6] |  |  | 0 | [10] |
Fair value measured on a recurring basis | Other long-term investments | Level 2 Inputs |  |  |  |  |  |  |
Investment securities |  |  |  |  |  |  |
Fair value investments | 0 | [6] |  |  | 0 | [10] |
Fair value measured on a recurring basis | Other long-term investments | Level 3 Inputs |  |  |  |  |  |  |
Investment securities |  |  |  |  |  |  |
Fair value investments | 269,200,000 | [6] |  |  | 259,300,000 | [10] |
Carrying value of investment accounted for using the equity method | 34,200,000 |  | 33,000,000 |  | 35,000,000 |  | 34,000,000
Fair value measured on a recurring basis | Other long-term investments | Fair Value |  |  |  |  |  |  |
Investment securities |  |  |  |  |  |  |
Fair value investments | 269,200,000 | [6] |  |  | 259,300,000 | [10] |
Fair value measured on a recurring basis | Forward Contracts | Level 2 Inputs |  |  |  |  |  |  |
Investment securities |  |  |  |  |  |  |
Realized and unrealized (losses) gains | (100,000) |  |  |  | (100,000) |  |
Fair value measured on a recurring basis | Short-term investments | Level 1 Inputs |  |  |  |  |  |  |
Investment securities |  |  |  |  |  |  |
Fair value investments | 630,600,000 |  |  |  |  |  |
Fair value investments | 582,100,000 |  |  |  | 630,600,000 |  |
Carrying value of investment accounted for using the equity method | 606,800,000 |  | 618,400,000 |  | 630,600,000 |  | 846,000,000
Realized and unrealized gain | 0 |  | 500,000 |  | 100,000 |  |
Fair value measured on a recurring basis | Short-term investments | Level 2 Inputs |  |  |  |  |  |  |
Investment securities |  |  |  |  |  |  |
Fair value investments | 24,700,000 |  |  |  | 0 |  |
Fair value measured on a recurring basis | Short-term investments | Level 3 Inputs |  |  |  |  |  |  |
Investment securities |  |  |  |  |  |  |
Fair value investments | 0 |  |  |  | 0 |  |
Fair value measured on a recurring basis | Short-term investments | Fair Value |  |  |  |  |  |  |
Investment securities |  |  |  |  |  |  |
Fair value investments | 606,800,000 |  |  |  | 630,600,000 |  |
Segment, Discontinued Operations |  |  |  |  |  |  |
Investment securities |  |  |  |  |  |  |
Fixed maturity investments reclassified to assets held for sale | (260,800,000) |  | 111,800,000 |  | 338,100,000 |  |
Prospector Offshore Fund Ltd [Member] | Fair value measured on a recurring basis | Short-term investments |  |  |  |  |  |  |
Investment securities |  |  |  |  |  |  |
Realized and unrealized gain | $ 0 |  | $ 0 |  |  |  |
[1] | Excludes carrying value of $630.6 and $606.8 at January 1, 2013 and June 30, 2013 and realized and unrealized gain for the periods of $0.1 and $0.8 associated with short-term investments classified.
[2] | Excludes carrying value of $846.0 and $618.4 January 1, 2012 and June 30, 2012 realized and unrealized loss for the period of $0.5 associated with short-term investment
[3] | ) Excludes $9.9 realized and unrealized losses associated with the Prospector Funds consolidation of investment-related liabilities.
[4] | Excludes $4.2 realized and unrealized losses associated with the Prospector Funds consolidation of investment-related liabilities.
[5] | Carrying value includes $260.8 that is classified as assets held for sale relating to discontinued operations.
[6] | Excludes carrying value of $34.2 associated with other long-term investment limited partnerships accounted for using the equity method and $(0.1) related to forward contracts.
[7] | Carrying value includes $111.8 that is classified as assets held for sale relating to discontinued operations.
[8] | Excludes carrying value of $33.0 and $34.0 at January 1, 2012 and June 30, 2012 associated with other long-term investment limited partnerships accounted for using the equity method.
[9] | Carrying value includes $338.1 that is classified as assets held for sale relating to discontinued operations.
[10] | Excludes carrying value of $35.0 associated with other long-term investment limited partnerships accounted for using the equity method and $(0.1) related to currency forward contracts.

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- Definition The sum of the periodic adjustments of the differences between securities' face values and purchase prices that are charged against earnings. This is called accretion if the security was purchased at a discount and amortization if it was purchased at premium. As a noncash item, this element is an adjustment to net income when calculating cash provided by or used in operations using the indirect method. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 28 -URI http://asc.fasb.org/extlink&oid=6943989&loc=d3e3602-108585 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 95 -Paragraph 28 -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
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- 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 investments accounted for under the equity method of accounting as of the balance sheet date. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher AICPA -Name Accounting Principles Board Opinion (APB) -Number 18 -Paragraph 6 -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 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 323 -SubTopic 10 -Section 25 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=6903542&loc=d3e32014-111567 + Details
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- 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 investment securities as of the balance sheet date which may include marketable securities, derivative financial instruments, and investments accounted for under the equity method. + References Reference 1: 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 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Available-for-Sale Securities -URI http://asc.fasb.org/extlink&oid=6505594 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 115 -Paragraph 12 -Subparagraph a -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 -Glossary Trading Securities -URI http://asc.fasb.org/extlink&oid=6526789 Reference 5: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 25 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=7534914&loc=d3e22054-111558 Reference 6: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 03 -Paragraph 1 -Subparagraph a, f, g -Article 7 Reference 7: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 35 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=16383099&loc=d3e24584-111560 + Details
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