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Rollforward of Fair Value Measurements by Level (Details) $ in Millions | 3 Months Ended | 9 Months Ended |  |
--- | --- | --- | --- | ---
Sep. 30, 2015 USD ($) | Sep. 30, 2014 USD ($) | Sep. 30, 2015 USD ($) | Sep. 30, 2014 USD ($) | Dec. 31, 2014 USD ($) | Dec. 31, 2013 USD ($)
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |  |  |
Balance at January 1, 2015 |  |  | $ 3,343.6 | $ 3,889.0 |  |
Total realized and unrealized (losses) gains |  |  | (32.7) | 49.7 |  |
Amortization/Accretion |  |  | (15.2) | (14.3) |  |
Purchases |  |  | 1,672.0 | 2,016.2 |  |
Sales |  |  | (1,540.9) | (2,340.7) |  |
Net change in investments related to (sales) purchases of consolidated affiliates |  |  |  | 10.8 |  |
Fair Value, Assets Measured on Recurring Basis, Other |  |  | (61.4) |  |  |
Transfers in |  |  | 41.7 | 42.1 |  |
Transfers out |  |  | (41.7) | (42.1) |  |
Balance at September 30, 2015 | $ 3,365.4 | $ 3,610.7 | 3,365.4 | 3,610.7 |  |
Other Investments | 294.6 |  | 294.6 |  | $ 318.0 |
Realized and unrealized gain | (34.5) | (21.9) | (27.0) | 52.2 |  |
Net realized and unrealized investment (losses) gains | (43.9) | (27.5) | (33.9) | 64.4 |  |
Short-term investments, at amortized cost (which approximates fair value) | 325.0 |  | $ 325.0 |  | 376.8 |
Maximum Percentage Differences Between Prices Provided by Pricing Services Considered Outliers |  |  | (5.00%) |  |  |
Maximum price difference provided by pricing services |  |  | $ 1.0 |  |  |
Level 1 Inputs |  |  |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |  |  |
Balance at January 1, 2015 |  |  | 550.6 | 997.2 |  |
Total realized and unrealized (losses) gains |  |  | (19.5) | 30.7 |  |
Amortization/Accretion |  |  | 0.0 | 0.1 |  |
Purchases |  |  | 550.0 | 661.2 |  |
Sales |  |  | (618.2) | (926.2) |  |
Net change in investments related to (sales) purchases of consolidated affiliates |  |  |  | 3.4 |  |
Fair Value, Assets Measured on Recurring Basis, Other |  |  | (43.5) |  |  |
Transfers in |  |  | 0.0 | 0.0 |  |
Transfers out |  |  | (1.2) | (0.2) |  |
Balance at September 30, 2015 | 418.2 | 766.2 | 418.2 | 766.2 |  |
Level 2 Inputs |  |  |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |  |  |
Balance at January 1, 2015 |  |  | 2,372.9 | 2,575.4 |  |
Total realized and unrealized (losses) gains |  |  | 0.6 | 12.5 |  |
Amortization/Accretion |  |  | (15.2) | (14.3) |  |
Purchases |  |  | 1,056.0 | 1,265.3 |  |
Sales |  |  | (894.4) | (1,392.5) |  |
Net change in investments related to (sales) purchases of consolidated affiliates |  |  |  | 7.4 |  |
Fair Value, Assets Measured on Recurring Basis, Other |  |  | 0.0 |  |  |
Transfers in |  |  | 41.7 | 42.1 |  |
Transfers out |  |  | 0.0 | 0.0 |  |
Balance at September 30, 2015 | 2,561.6 | 2,495.9 | 2,561.6 | 2,495.9 |  |
Fixed maturity investments. | Level 3 Inputs |  |  |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |  |  |
Balance at January 1, 2015 |  |  | 76.4 | 81.8 |  |
Total realized and unrealized (losses) gains |  |  | (0.9) | 2.7 |  |
Amortization/Accretion |  |  | 0.0 | (0.1) |  |
Purchases |  |  | 35.3 | 45.5 |  |
Sales |  |  | 0.0 | 0.0 |  |
Net change in investments related to (sales) purchases of consolidated affiliates |  |  |  | 0.0 |  |
Fair Value, Assets Measured on Recurring Basis, Other |  |  | 0.0 |  |  |
Transfers in |  |  | 0.0 | 0.0 |  |
Transfers out |  |  | (40.5) | (41.9) |  |
Balance at September 30, 2015 | 70.3 | $ 88.0 | 70.3 | $ 88.0 |  |
Number of Investments |  | 6 |  | 6 |  |
Common equity securities | Level 3 Inputs |  |  |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |  |  |
Balance at January 1, 2015 |  |  | 39.5 | $ 45.4 |  |
Total realized and unrealized (losses) gains |  |  | 7.7 | 3.7 |  |
Amortization/Accretion |  |  | 0.0 | 0.0 |  |
Purchases |  |  | 0.0 | 0.0 |  |
Sales |  |  | (9.4) | (10.9) |  |
Net change in investments related to (sales) purchases of consolidated affiliates |  |  |  | 0.0 |  |
Fair Value, Assets Measured on Recurring Basis, Other |  |  | (3.6) |  |  |
Transfers in |  |  | 0.0 | 0.0 |  |
Transfers out |  |  | 0.0 | 0.0 |  |
Balance at September 30, 2015 | 34.2 | $ 38.2 | 34.2 | 38.2 |  |
Convertible Debt Securities | Level 3 Inputs |  |  |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |  |  |
Balance at January 1, 2015 |  |  | 8.2 | 6.1 |  |
Total realized and unrealized (losses) gains |  |  | (2.6) | 3.3 |  |
Amortization/Accretion |  |  | 0.0 | 0.0 |  |
Purchases |  |  | 0.0 | 1.5 |  |
Sales |  |  | 0.0 | 0.0 |  |
Net change in investments related to (sales) purchases of consolidated affiliates |  |  |  | 0.0 |  |
Fair Value, Assets Measured on Recurring Basis, Other |  |  | 0.0 |  |  |
Transfers in |  |  | 0.0 | 0.0 |  |
Transfers out |  |  | 0.0 | 0.0 |  |
Balance at September 30, 2015 | 5.6 | 10.9 | 5.6 | 10.9 |  |
Other long-term investments | Level 3 Inputs |  |  |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |  |  |
Balance at January 1, 2015 |  |  | 296.0 | 183.1 |  |
Total realized and unrealized (losses) gains |  |  | (18.0) | (3.2) |  |
Amortization/Accretion |  |  | 0.0 | 0.0 |  |
Purchases |  |  | 30.7 | 42.7 |  |
Sales |  |  | (18.9) | (11.1) |  |
Net change in investments related to (sales) purchases of consolidated affiliates |  |  |  | 0.0 |  |
Fair Value, Assets Measured on Recurring Basis, Other |  |  | (14.3) |  |  |
Transfers in |  |  | 0.0 | 0.0 |  |
Transfers out |  |  | 0.0 | 0.0 |  |
Balance at September 30, 2015 | 275.5 | 211.5 | 275.5 | 211.5 |  |
Fair value measured on a recurring basis |  |  |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |  |  |
Balance at January 1, 2015 |  |  | 3,720.4 |  |  |
Balance at September 30, 2015 | 3,690.4 |  | 3,690.4 |  |  |
Fair value measured on a recurring basis | Level 1 Inputs |  |  |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |  |  |
Balance at January 1, 2015 |  |  | 926.9 |  |  |
Balance at September 30, 2015 | 743.2 |  | 743.2 |  |  |
Fair value measured on a recurring basis | Level 2 Inputs |  |  |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |  |  |
Balance at January 1, 2015 |  |  | 2,373.4 |  |  |
Balance at September 30, 2015 | 2,561.6 |  | 2,561.6 |  |  |
Fair value measured on a recurring basis | Level 3 Inputs |  |  |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |  |  |
Balance at January 1, 2015 |  |  | 420.1 |  |  |
Balance at September 30, 2015 | 385.6 |  | 385.6 |  |  |
Fair value measured on a recurring basis | Fixed maturity investments. |  |  |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |  |  |
Balance at January 1, 2015 |  |  | 2,422.0 |  |  |
Balance at September 30, 2015 | 2,602.2 |  | 2,602.2 |  |  |
Fair value measured on a recurring basis | Fixed maturity investments. | Level 1 Inputs |  |  |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |  |  |
Balance at January 1, 2015 |  |  | 51.8 |  |  |
Balance at September 30, 2015 | 62.4 |  | 62.4 |  |  |
Fair value measured on a recurring basis | Fixed maturity investments. | Level 2 Inputs |  |  |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |  |  |
Balance at January 1, 2015 |  |  | 2,293.8 |  |  |
Balance at September 30, 2015 | 2,469.5 |  | 2,469.5 |  |  |
Fair value measured on a recurring basis | Fixed maturity investments. | Level 3 Inputs |  |  |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |  |  |
Balance at January 1, 2015 |  |  | 76.4 |  |  |
Balance at September 30, 2015 | 70.3 |  | 70.3 |  |  |
Fair value measured on a recurring basis | Convertible Debt Securities |  |  |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |  |  |
Balance at January 1, 2015 |  |  | 13.9 |  |  |
Balance at September 30, 2015 | 5.6 |  | 5.6 |  |  |
Fair value measured on a recurring basis | Convertible Debt Securities | Level 1 Inputs |  |  |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |  |  |
Balance at January 1, 2015 |  |  | 0.0 |  |  |
Balance at September 30, 2015 | 0.0 |  | 0.0 |  |  |
Fair value measured on a recurring basis | Convertible Debt Securities | Level 2 Inputs |  |  |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |  |  |
Balance at January 1, 2015 |  |  | 5.7 |  |  |
Balance at September 30, 2015 | 0.0 |  | 0.0 |  |  |
Fair value measured on a recurring basis | Convertible Debt Securities | Level 3 Inputs |  |  |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |  |  |
Balance at January 1, 2015 |  |  | 8.2 |  |  |
Balance at September 30, 2015 | 5.6 |  | 5.6 |  |  |
Fair value measured on a recurring basis | Other long-term investments |  |  |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |  |  |
Balance at January 1, 2015 |  |  | 296.0 |  |  |
Balance at September 30, 2015 | 275.5 |  | 275.5 |  |  |
Carrying value of investment accounted for using the equity method | 3.9 |  | 3.9 |  |  |
Fair value measured on a recurring basis | Other long-term investments | Level 1 Inputs |  |  |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |  |  |
Balance at January 1, 2015 |  |  | 0.0 |  |  |
Balance at September 30, 2015 | 0.0 |  | 0.0 |  |  |
Fair value measured on a recurring basis | Other long-term investments | Level 2 Inputs |  |  |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |  |  |
Balance at January 1, 2015 |  |  | 0.0 |  |  |
Balance at September 30, 2015 | 0.0 |  | 0.0 |  |  |
Fair value measured on a recurring basis | Other long-term investments | Level 3 Inputs |  |  |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |  |  |
Balance at January 1, 2015 |  |  | 296.0 |  |  |
Balance at September 30, 2015 | 275.5 |  | 275.5 |  |  |
Carrying value of investment accounted for using the equity method |  | 5.1 |  | 5.1 | 5.2 | $ 6.8
Fair value measured on a recurring basis | Short-term investments |  |  |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |  |  |
Balance at January 1, 2015 |  |  | 376.8 |  |  |
Balance at September 30, 2015 | 325.0 |  | 325.0 |  |  |
Fair value measured on a recurring basis | Short-term investments | Level 1 Inputs |  |  |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |  |  |
Balance at January 1, 2015 |  |  | 376.3 |  |  |
Balance at September 30, 2015 | 325.0 |  | 325.0 |  |  |
Short-term investments, at amortized cost (which approximates fair value) | 325.0 | 402.7 | 325.0 | 402.7 | 376.8 | 310.4
Fair value measured on a recurring basis | Short-term investments | Level 2 Inputs |  |  |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |  |  |
Balance at January 1, 2015 |  |  | 0.5 |  |  |
Balance at September 30, 2015 | 0.0 |  | 0.0 |  |  |
Fair value measured on a recurring basis | Short-term investments | Level 3 Inputs |  |  |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |  |  |
Balance at January 1, 2015 |  |  | 0.0 |  |  |
Balance at September 30, 2015 | 0.0 |  | 0.0 |  |  |
Discontinued Operations [Member] |  |  |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |  |  |
Net realized and unrealized investment (losses) gains | (0.9) | 48.3 | 31.4 | 134.1 |  |
Fixed maturity investments reclassified to assets held for sale |  | 203.9 |  | 203.9 |  | 236.3
Prospector Offshore Fund Ltd [Member] | Fair value measured on a recurring basis | Investment related liabilities [Member] |  |  |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |  |  |
Net realized and unrealized investment (losses) gains |  |  | 0.8 | 0.5 |  |
Affordable Housing Development Fund [Member] |  |  |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |  |  |
Other Investments | 15.2 | $ 17.3 | 15.2 | $ 17.3 | 16.8 | $ 19.1
Affordable Housing Development Fund [Member] | Fair value measured on a recurring basis | Other long-term investments |  |  |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |  |  |
Other Investments | $ 15.2 |  | $ 15.2 |  | $ 16.8 |

X
- 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=56944662&loc=d3e3602-108585 + Details
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- Definition Fair value portion of investments accounted under the equity method. + References Reference 1: 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=35740979&loc=d3e32014-111567 + Details
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- Definition This item represents the net total realized and unrealized gain (loss) included in earnings for the period as a result of selling or holding marketable securities categorized as trading, available-for-sale, or held-to-maturity, including the unrealized holding gain (loss) of held-to-maturity securities transferred to the trading security category and the cumulative unrealized gain (loss) which was included in other comprehensive income (a separate component of shareholders' equity) for available-for-sale securities transferred to trading securities during the period. Additionally, this item would include any gains (losses) realized during the period from the sale of investments accounted for under the cost method of accounting and losses recognized for other than temporary impairments (OTTI) of the subject investments. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 9 -Subparagraph (c)-(e) -URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27357-111563 Reference 2: 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=56944662&loc=d3e3602-108585 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 225 -SubTopic 10 -Section S99 -Paragraph 2 -Subparagraph (SX 210.5-03.7(c),9(a)) -URI http://asc.fasb.org/extlink&oid=26872669&loc=d3e20235-122688 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 10 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27405-111563 + Details
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- Definition Fair value portion of investment securities, including, but not limited to, 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 Accounting Standards Codification -Glossary Trading Securities -URI http://asc.fasb.org/extlink&oid=6526789 Reference 2: 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=27723805&loc=d3e24584-111560 Reference 3: 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=51813488&loc=d3e22054-111558 Reference 4: 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 5: 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 + Details
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- Definition Other investments not otherwise specified in the taxonomy. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.12) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-03.1(f,g)) -URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910 + Details
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- Definition The cash outflow associated with the purchase of all investments (debt, security, other) during the period. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Investing Activities -URI http://asc.fasb.org/extlink&oid=6516133 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 13 -URI http://asc.fasb.org/extlink&oid=56944662&loc=d3e3213-108585 + Details
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- Definition The cash inflow associated with the sale, maturities, repayments and calls of securities classified as trading securities and held for investment purposes. Excludes proceeds from trading securities purchased and held principally for the purpose of selling them in the near term (thus held for only a short period of time). + References No definition available. + Details
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- Definition Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table. + References No definition available. + Details
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- Definition Investments which are intended to be sold in the short term (usually less than one year or the normal operating cycle, whichever is longer) including trading securities, available-for-sale securities, held-to-maturity securities, and other short-term investments not otherwise listed in the existing taxonomy. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.2) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 03 -Paragraph 1 -Subparagraph g -Article 7 + Details
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- Definition This element represents the amount of the total realized and unrealized (holding) gains or losses for the period which are included in the statement of income. + References No definition available. + Details
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- Definition This element represents transfers in to one of the levels in the fair value hierarchy for assets measured at fair value on a recurring basis. + References No definition available. + Details
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- Definition This element represents transfers out of one of the levels in the fair value hierarchy for assets measured at fair value on a recurring basis. + References No definition available. + Details
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- Definition This element represents other reconciling items in the fair value hierarchy for assets measured at fair value on a recurring basis. + References No definition available. + Details
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- Definition Represents the maximum price difference provided by the pricing services between the market price and the actual sale price of securities, on the basis of which outliers are considered. + References No definition available. + Details
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- Definition Represents the maximum percentage of price difference provided by the pricing services between the market price and the actual sale price of securities, on the basis of which outliers are considered. + References No definition available. + Details
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- Definition Net change in investments related to purchases and sales of consolidated/unconsolidated affiliates + References No definition available. + Details
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- Definition Net Realized and Unrealized Gain (Loss) on Trading Securities, Net of Tax + References No definition available. + Details
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- Definition Number of Investments + References No definition available. + Details
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- Definition Represents the fair value of trading debt securities re-classified to assets held for sale. + References No definition available. + Details
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