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Note 4 - Fair Value of Financial Instruments Fair Value Assets Liabilities by Hierarchy Level (Details) - USD ($) | 3 Months Ended | 6 Months Ended |
--- | --- | --- | ---
Jun. 30, 2015 | Jun. 30, 2014 | Jun. 30, 2015 | Jun. 30, 2014 | Dec. 31, 2014
Carrying (Reported) Amount, Fair Value Disclosure [Member] |  |  |  |  |  |  |  |  |
Fair Value by Hierarchy Level [Line Items] |  |  |  |  |  |  |  |  |
Other Invested Assets, Carrying Amount |  | $ 13,600,000 |  |  | $ 13,600,000 |  |  | $ 14,600,000 |
Fair Value, Measurements, Recurring |  |  |  |  |  |  |  |  |
Fair Value by Hierarchy Level [Line Items] |  |  |  |  |  |  |  |  |
Total Investments at Fair Value |  | 4,295,500,000 | [1] |  | 4,295,500,000 | [1] |  | 3,614,700,000 | [2]
Total Assets at Fair Value |  | 4,295,500,000 |  |  | 4,295,500,000 |  |  | 3,614,700,000 |
Fair Value, Assets, Level 2 to Level 1 Transfers, Amount |  | 0 |  | $ 0 | 0 |  | $ 0 |  |
Fair Value, Assets, Level 1 to Level 2 Transfers, Amount |  | 0 |  | 0 | 0 |  | 0 |  |
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers, Net |  | 0 |  |  | 0 |  |  |  |
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Transfers, Net |  | 0 |  |  | 0 |  |  |  |
US government and agency securities | Fair Value, Measurements, Recurring |  |  |  |  |  |  |  |  |
Fair Value by Hierarchy Level [Line Items] |  |  |  |  |  |  |  |  |
Total Investments at Fair Value |  | 598,300,000 |  |  | 598,300,000 |  |  | 839,900,000 |
State and municipal obligations | Fair Value, Measurements, Recurring |  |  |  |  |  |  |  |  |
Fair Value by Hierarchy Level [Line Items] |  |  |  |  |  |  |  |  |
Total Investments at Fair Value |  | 330,800,000 |  |  | 330,800,000 |  |  | 362,800,000 |
Money market instruments | Fair Value, Measurements, Recurring |  |  |  |  |  |  |  |  |
Fair Value by Hierarchy Level [Line Items] |  |  |  |  |  |  |  |  |
Total Investments at Fair Value |  | 828,100,000 |  |  | 828,100,000 |  |  | 600,300,000 |
Corporate bonds and notes | Fair Value, Measurements, Recurring |  |  |  |  |  |  |  |  |
Fair Value by Hierarchy Level [Line Items] |  |  |  |  |  |  |  |  |
Total Investments at Fair Value |  | 1,321,700,000 |  |  | 1,321,700,000 |  |  | 992,800,000 |
RMBS | Fair Value, Measurements, Recurring |  |  |  |  |  |  |  |  |
Fair Value by Hierarchy Level [Line Items] |  |  |  |  |  |  |  |  |
Total Investments at Fair Value |  | 237,300,000 |  |  | 237,300,000 |  |  | 132,300,000 |
CMBS | Fair Value, Measurements, Recurring |  |  |  |  |  |  |  |  |
Fair Value by Hierarchy Level [Line Items] |  |  |  |  |  |  |  |  |
Total Investments at Fair Value |  | 383,400,000 |  |  | 383,400,000 |  |  | 246,800,000 |
Other ABS | Fair Value, Measurements, Recurring |  |  |  |  |  |  |  |  |
Fair Value by Hierarchy Level [Line Items] |  |  |  |  |  |  |  |  |
Total Investments at Fair Value |  | 274,000,000 |  |  | 274,000,000 |  |  | 185,500,000 |
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Transfers Into Level 3 |  |  |  | $ 17,300,000 |  |  | $ 17,300,000 |  |
Foreign government securities | Fair Value, Measurements, Recurring |  |  |  |  |  |  |  |  |
Fair Value by Hierarchy Level [Line Items] |  |  |  |  |  |  |  |  |
Total Investments at Fair Value |  | 36,700,000 |  |  | 36,700,000 |  |  | 37,700,000 |
Equity securities | Fair Value, Measurements, Recurring |  |  |  |  |  |  |  |  |
Fair Value by Hierarchy Level [Line Items] |  |  |  |  |  |  |  |  |
Total Investments at Fair Value |  | 68,000,000 |  |  | 68,000,000 |  |  | 215,600,000 | [3]
Other investments | Fair Value, Measurements, Recurring |  |  |  |  |  |  |  |  |
Fair Value by Hierarchy Level [Line Items] |  |  |  |  |  |  |  |  |
Total Investments at Fair Value |  | 217,200,000 | [4] |  | 217,200,000 | [4] |  | 1,000,000 | [5]
Fair Value, Inputs, Level 1 | Fair Value, Measurements, Recurring |  |  |  |  |  |  |  |  |
Fair Value by Hierarchy Level [Line Items] |  |  |  |  |  |  |  |  |
Total Investments at Fair Value |  | 1,447,500,000 | [1] |  | 1,447,500,000 | [1] |  | 1,601,200,000 | [2]
Total Assets at Fair Value |  | 1,447,500,000 |  |  | 1,447,500,000 |  |  | 1,601,200,000 |
Assets Held-for-sale, Long Lived, Fair Value Disclosure |  |  |  |  |  |  |  | 495,100,000 |
Fair Value, Inputs, Level 1 | US government and agency securities | Fair Value, Measurements, Recurring |  |  |  |  |  |  |  |  |
Fair Value by Hierarchy Level [Line Items] |  |  |  |  |  |  |  |  |
Total Investments at Fair Value |  | 598,300,000 |  |  | 598,300,000 |  |  | 836,900,000 |
Fair Value, Inputs, Level 1 | State and municipal obligations | Fair Value, Measurements, Recurring |  |  |  |  |  |  |  |  |
Fair Value by Hierarchy Level [Line Items] |  |  |  |  |  |  |  |  |
Total Investments at Fair Value |  | 0 |  |  | 0 |  |  | 0 |
Fair Value, Inputs, Level 1 | Money market instruments | Fair Value, Measurements, Recurring |  |  |  |  |  |  |  |  |
Fair Value by Hierarchy Level [Line Items] |  |  |  |  |  |  |  |  |
Total Investments at Fair Value |  | 828,100,000 |  |  | 828,100,000 |  |  | 600,300,000 |
Fair Value, Inputs, Level 1 | Corporate bonds and notes | Fair Value, Measurements, Recurring |  |  |  |  |  |  |  |  |
Fair Value by Hierarchy Level [Line Items] |  |  |  |  |  |  |  |  |
Total Investments at Fair Value |  | 0 |  |  | 0 |  |  | 0 |
Fair Value, Inputs, Level 1 | RMBS | Fair Value, Measurements, Recurring |  |  |  |  |  |  |  |  |
Fair Value by Hierarchy Level [Line Items] |  |  |  |  |  |  |  |  |
Total Investments at Fair Value |  | 0 |  |  | 0 |  |  | 0 |
Fair Value, Inputs, Level 1 | CMBS | Fair Value, Measurements, Recurring |  |  |  |  |  |  |  |  |
Fair Value by Hierarchy Level [Line Items] |  |  |  |  |  |  |  |  |
Total Investments at Fair Value |  | 0 |  |  | 0 |  |  | 0 |
Fair Value, Inputs, Level 1 | Other ABS | Fair Value, Measurements, Recurring |  |  |  |  |  |  |  |  |
Fair Value by Hierarchy Level [Line Items] |  |  |  |  |  |  |  |  |
Total Investments at Fair Value |  | 0 |  |  | 0 |  |  | 0 |
Fair Value, Inputs, Level 1 | Foreign government securities | Fair Value, Measurements, Recurring |  |  |  |  |  |  |  |  |
Fair Value by Hierarchy Level [Line Items] |  |  |  |  |  |  |  |  |
Total Investments at Fair Value |  | 0 |  |  | 0 |  |  | 0 |
Fair Value, Inputs, Level 1 | Equity securities | Fair Value, Measurements, Recurring |  |  |  |  |  |  |  |  |
Fair Value by Hierarchy Level [Line Items] |  |  |  |  |  |  |  |  |
Total Investments at Fair Value |  | 21,100,000 |  |  | 21,100,000 |  |  | 164,000,000 | [3]
Fair Value, Inputs, Level 1 | Other investments | Fair Value, Measurements, Recurring |  |  |  |  |  |  |  |  |
Fair Value by Hierarchy Level [Line Items] |  |  |  |  |  |  |  |  |
Total Investments at Fair Value |  | 0 | [4] |  | 0 | [4] |  | 0 | [5]
Fair Value, Inputs, Level 2 | Fair Value, Measurements, Recurring |  |  |  |  |  |  |  |  |
Fair Value by Hierarchy Level [Line Items] |  |  |  |  |  |  |  |  |
Total Investments at Fair Value |  | 2,847,500,000 | [1] |  | 2,847,500,000 | [1] |  | 2,013,500,000 | [2]
Total Assets at Fair Value |  | 2,847,500,000 |  |  | 2,847,500,000 |  |  | 2,013,500,000 |
Assets Held-for-sale, Long Lived, Fair Value Disclosure |  |  |  |  |  |  |  | 839,200,000 |
Fair Value, Inputs, Level 2 | US government and agency securities | Fair Value, Measurements, Recurring |  |  |  |  |  |  |  |  |
Fair Value by Hierarchy Level [Line Items] |  |  |  |  |  |  |  |  |
Total Investments at Fair Value |  | 0 |  |  | 0 |  |  | 3,000,000 |
Fair Value, Inputs, Level 2 | State and municipal obligations | Fair Value, Measurements, Recurring |  |  |  |  |  |  |  |  |
Fair Value by Hierarchy Level [Line Items] |  |  |  |  |  |  |  |  |
Total Investments at Fair Value |  | 330,800,000 |  |  | 330,800,000 |  |  | 362,800,000 |
Fair Value, Inputs, Level 2 | Money market instruments | Fair Value, Measurements, Recurring |  |  |  |  |  |  |  |  |
Fair Value by Hierarchy Level [Line Items] |  |  |  |  |  |  |  |  |
Total Investments at Fair Value |  | 0 |  |  | 0 |  |  | 0 |
Fair Value, Inputs, Level 2 | Corporate bonds and notes | Fair Value, Measurements, Recurring |  |  |  |  |  |  |  |  |
Fair Value by Hierarchy Level [Line Items] |  |  |  |  |  |  |  |  |
Total Investments at Fair Value |  | 1,321,700,000 |  |  | 1,321,700,000 |  |  | 992,800,000 |
Fair Value, Inputs, Level 2 | RMBS | Fair Value, Measurements, Recurring |  |  |  |  |  |  |  |  |
Fair Value by Hierarchy Level [Line Items] |  |  |  |  |  |  |  |  |
Total Investments at Fair Value |  | 237,300,000 |  |  | 237,300,000 |  |  | 132,300,000 |
Fair Value, Inputs, Level 2 | CMBS | Fair Value, Measurements, Recurring |  |  |  |  |  |  |  |  |
Fair Value by Hierarchy Level [Line Items] |  |  |  |  |  |  |  |  |
Total Investments at Fair Value |  | 383,400,000 |  |  | 383,400,000 |  |  | 246,800,000 |
Fair Value, Inputs, Level 2 | Other ABS | Fair Value, Measurements, Recurring |  |  |  |  |  |  |  |  |
Fair Value by Hierarchy Level [Line Items] |  |  |  |  |  |  |  |  |
Total Investments at Fair Value |  | 274,000,000 |  |  | 274,000,000 |  |  | 185,500,000 |
Fair Value, Inputs, Level 2 | Foreign government securities | Fair Value, Measurements, Recurring |  |  |  |  |  |  |  |  |
Fair Value by Hierarchy Level [Line Items] |  |  |  |  |  |  |  |  |
Total Investments at Fair Value |  | 36,700,000 |  |  | 36,700,000 |  |  | 37,700,000 |
Fair Value, Inputs, Level 2 | Equity securities | Fair Value, Measurements, Recurring |  |  |  |  |  |  |  |  |
Fair Value by Hierarchy Level [Line Items] |  |  |  |  |  |  |  |  |
Total Investments at Fair Value |  | 46,400,000 |  |  | 46,400,000 |  |  | 51,600,000 | [3]
Fair Value, Inputs, Level 2 | Other investments | Fair Value, Measurements, Recurring |  |  |  |  |  |  |  |  |
Fair Value by Hierarchy Level [Line Items] |  |  |  |  |  |  |  |  |
Total Investments at Fair Value |  | 217,200,000 | [4] |  | 217,200,000 | [4] |  | 1,000,000 | [5]
Fair Value, Inputs, Level 2 | Commercial Paper | Fair Value, Measurements, Recurring |  |  |  |  |  |  |  |  |
Fair Value by Hierarchy Level [Line Items] |  |  |  |  |  |  |  |  |
Total Investments at Fair Value |  | 216,200,000 |  |  | 216,200,000 |  |  |  |
Fair Value, Inputs, Level 2 | Short Term CDs | Fair Value, Measurements, Recurring |  |  |  |  |  |  |  |  |
Fair Value by Hierarchy Level [Line Items] |  |  |  |  |  |  |  |  |
Total Investments at Fair Value |  | 1,000,000 |  |  | 1,000,000 |  |  |  |
Fair Value, Inputs, Level 3 | Fair Value, Measurements, Recurring |  |  |  |  |  |  |  |  |
Fair Value by Hierarchy Level [Line Items] |  |  |  |  |  |  |  |  |
Total Investments at Fair Value | [1] | 500,000 |  |  | 500,000 |  |  |  |
Total Assets at Fair Value |  | 500,000 |  |  | 500,000 |  |  | 0 |
Liabilities, Fair Value Disclosure, Recurring |  | 0 |  |  | 0 |  |  |  |
Assets Held-for-sale, Long Lived, Fair Value Disclosure |  |  |  |  |  |  |  | 102,600,000 |
Variable Interest Entity Debt |  |  |  |  |  |  |  | $ 3,800,000 |
Total Level III Liabilities |  |  |  |  |  |  |  | 100.00% |
Fair Value, Inputs, Level 3 | US government and agency securities | Fair Value, Measurements, Recurring |  |  |  |  |  |  |  |  |
Fair Value by Hierarchy Level [Line Items] |  |  |  |  |  |  |  |  |
Total Investments at Fair Value |  | 0 |  |  | 0 |  |  |  |
Fair Value, Inputs, Level 3 | State and municipal obligations | Fair Value, Measurements, Recurring |  |  |  |  |  |  |  |  |
Fair Value by Hierarchy Level [Line Items] |  |  |  |  |  |  |  |  |
Total Investments at Fair Value |  | 0 |  |  | 0 |  |  |  |
Fair Value, Inputs, Level 3 | Money market instruments | Fair Value, Measurements, Recurring |  |  |  |  |  |  |  |  |
Fair Value by Hierarchy Level [Line Items] |  |  |  |  |  |  |  |  |
Total Investments at Fair Value |  | 0 |  |  | 0 |  |  |  |
Fair Value, Inputs, Level 3 | Corporate bonds and notes | Fair Value, Measurements, Recurring |  |  |  |  |  |  |  |  |
Fair Value by Hierarchy Level [Line Items] |  |  |  |  |  |  |  |  |
Total Investments at Fair Value |  | 0 |  |  | 0 |  |  |  |
Fair Value, Inputs, Level 3 | RMBS | Fair Value, Measurements, Recurring |  |  |  |  |  |  |  |  |
Fair Value by Hierarchy Level [Line Items] |  |  |  |  |  |  |  |  |
Total Investments at Fair Value |  | 0 |  |  | 0 |  |  |  |
Fair Value, Inputs, Level 3 | CMBS | Fair Value, Measurements, Recurring |  |  |  |  |  |  |  |  |
Fair Value by Hierarchy Level [Line Items] |  |  |  |  |  |  |  |  |
Total Investments at Fair Value |  | 0 |  |  | 0 |  |  |  |
Fair Value, Inputs, Level 3 | Other ABS | Fair Value, Measurements, Recurring |  |  |  |  |  |  |  |  |
Fair Value by Hierarchy Level [Line Items] |  |  |  |  |  |  |  |  |
Total Investments at Fair Value |  | 0 |  |  | 0 |  |  |  |
Fair Value, Inputs, Level 3 | Foreign government securities | Fair Value, Measurements, Recurring |  |  |  |  |  |  |  |  |
Fair Value by Hierarchy Level [Line Items] |  |  |  |  |  |  |  |  |
Total Investments at Fair Value |  | 0 |  |  | 0 |  |  |  |
Fair Value, Inputs, Level 3 | Equity securities | Fair Value, Measurements, Recurring |  |  |  |  |  |  |  |  |
Fair Value by Hierarchy Level [Line Items] |  |  |  |  |  |  |  |  |
Total Investments at Fair Value |  | 500,000 |  |  | 500,000 |  |  |  |
Total Assets at Fair Value |  | 500,000 |  |  | 500,000 |  |  |  |
Gain (Loss) on Investments |  | 0 |  |  |  |  |  |  |
Fair Value, Inputs, Level 3 | Other investments | Fair Value, Measurements, Recurring |  |  |  |  |  |  |  |  |
Fair Value by Hierarchy Level [Line Items] |  |  |  |  |  |  |  |  |
Total Investments at Fair Value | [4] | $ 0 |  |  | $ 0 |  |  |  |
Maximum [Member] | Fair Value, Inputs, Level 3 | Fair Value, Measurements, Recurring |  |  |  |  |  |  |  |  |
Fair Value by Hierarchy Level [Line Items] |  |  |  |  |  |  |  |  |
Total Level III Assets as a Percentage of Total Assets Measured at Fair Value |  | 0.10% |  |  | 0.10% |  |  |  |
[1] | Does not include certain other invested assets ($13.6 million), primarily invested in limited partnerships, accounted for as cost-method investments and not measured at fair value.
[2] | Does not include certain other invested assets ($14.6 million), primarily invested in limited partnerships, accounted for as cost-method investments and not measured at fair value. Also excludes investments classified as assets held for sale of $495.1 million, $839.2 million and $102.6 million, with fair values categorized in Level I, Level II and Level III, respectively.
[3] | Comprising broadly diversified domestic equity mutual funds and certain common stocks included within Level I and various preferred stocks invested across numerous companies and industries included within Level II.
[4] | Comprising short-term certificates of deposit ($1.0 million) and short-term commercial paper ($216.2 million) included within Level II.
[5] | Comprising short-term certificates of deposit.

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- Definition Aggregate carrying amount, as of the balance sheet date, of other invested assets not separately disclosed in the balance sheet. + References No definition available. + Details
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- Definition The percentage of total level III assets to total assets measured at fair value. + References No definition available. + Details
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- Definition Total Level III Liabilities is the percentage of total level III liabilities to the total liabilities measured at fair value + References No definition available. + Details
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- Definition Fair value portion of probable future economic benefits obtained or controlled by an entity as a result of past transactions or events. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=36462937&loc=d3e19207-110258 + Details
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- Definition Fair value portion of assets classified as held for sale. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 1 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=36462937&loc=d3e19190-110258 + Details
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- Definition Amount of transfers of assets measured on a recurring basis out of Level 1 of the fair value hierarchy into Level 2. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (bb) -URI http://asc.fasb.org/extlink&oid=36462937&loc=d3e19207-110258 + Details
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- Definition Amount of transfers of assets measured on a recurring basis out of Level 2 of the fair value hierarchy into Level 1. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (bb) -URI http://asc.fasb.org/extlink&oid=36462937&loc=d3e19207-110258 + 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 Amount of transfer of financial instrument classified as an asset into level 3 of the fair value hierarchy. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c)(3) -URI http://asc.fasb.org/extlink&oid=36462937&loc=d3e19207-110258 + Details
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- Definition Amount of transfer of financial instrument classified as an asset into (out of) level 3 of the fair value hierarchy. + References No definition available. + Details
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- Definition Amount of transfers of financial instrument classified as a liability into (out of) level 3 of the fair value hierarchy. + References No definition available. + Details
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- Definition Fair value of financial instrument classified as a liability measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=36462937&loc=d3e19207-110258 + 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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