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Intangible Assets - Additional Information (Narrative) (Details) - USD ($) $ in Thousands |  |  |  |  |  | 12 Months Ended |  |
--- | --- | --- | --- | --- | --- | --- | --- | ---
Dec. 18, 2017 | Oct. 31, 2016 | Sep. 22, 2016 | Jan. 19, 2016 | Apr. 05, 2005 | Dec. 31, 2017 | Dec. 31, 2016 | Jan. 01, 2017 | Dec. 31, 2015
Finite And Indefinite Lived Intangible Assets By Major Class [Line Items] |  |  |  |  |  |  |  |  |
Goodwill, acquired during period |  |  |  |  |  | $ 1,800 |  |  |
Goodwill and intangible asset impairment |  |  |  |  |  | (3,400) | $ 0 |  |
AgriLogic |  |  |  |  |  |  |  |  |
Finite And Indefinite Lived Intangible Assets By Major Class [Line Items] |  |  |  |  |  |  |  |  |
Percentage of interests acquired |  |  |  | 100.00% |  |  |  |  |
Initial purchase price |  |  |  | $ 53,000 |  |  |  |  |
Business combination, contingent consideration, liability |  |  |  | 14,100 |  |  |  |  |
Business combination, contingent consideration, maximum payable |  |  |  | 22,800 |  |  |  |  |
Business combination, ceding commission |  |  |  | 2,000 |  |  |  |  |
Aggregate consideration for sale and purchase agreement |  |  |  | 69,100 |  |  |  |  |
Business combination, office properties and equipments |  |  |  | 12,000 |  |  |  |  |
Business combination, residual assets |  |  |  | 300 |  |  |  |  |
Business combination, net assets acquired |  |  |  | 47,000 |  |  |  |  |
Goodwill, acquired during period |  |  |  | 22,100 |  |  |  |  |
Intangible assets acquired eligible tax deductible amount |  |  |  | 34,000 |  |  |  |  |
Goodwill acquired eligible tax deductible amount |  |  |  | $ 21,000 |  |  |  |  |
Business acquisition, eligible tax deduction period |  |  |  | 15 years |  |  |  |  |
Blue Waters |  |  |  |  |  |  |  |  |
Finite And Indefinite Lived Intangible Assets By Major Class [Line Items] |  |  |  |  |  |  |  |  |
Percentage of interests acquired |  | 100.00% |  |  |  |  |  |  |
Aggregate consideration for sale and purchase agreement |  | $ 8,000 |  |  |  |  |  |  |
Business combination, residual assets |  | 300 |  |  |  |  |  |  |
Business combination, net assets acquired |  | 5,750 |  |  |  |  |  |  |
Goodwill, acquired during period |  | 2,100 |  |  |  |  |  |  |
Trade Mark |  |  |  |  |  |  |  |  |
Finite And Indefinite Lived Intangible Assets By Major Class [Line Items] |  |  |  |  |  |  |  |  |
Value of the asset |  |  |  |  |  | $ 2,900 | 6,600 |  | $ 1,600
Intangible Assets acquired (disposed) | $ (3,100) |  |  |  |  |  | 5,500 |  |
Trade Mark | AgriLogic |  |  |  |  |  |  |  |  |
Finite And Indefinite Lived Intangible Assets By Major Class [Line Items] |  |  |  |  |  |  |  |  |
Value of the asset |  |  |  | $ 4,000 |  |  |  |  |
Asset amortization period |  |  |  |  |  | 10 years |  |  |
Trade Mark | Blue Waters |  |  |  |  |  |  |  |  |
Finite And Indefinite Lived Intangible Assets By Major Class [Line Items] |  |  |  |  |  |  |  |  |
Value of the asset |  | 1,500 |  |  |  |  |  |  |
Asset amortization period |  |  |  |  |  | 5 years |  |  |
Aspen trademark |  |  |  |  |  |  |  |  |
Finite And Indefinite Lived Intangible Assets By Major Class [Line Items] |  |  |  |  |  |  |  |  |
Value of the asset |  |  |  |  |  | $ 1,600 |  |  |
Insurance Licenses |  |  |  |  |  |  |  |  |
Finite And Indefinite Lived Intangible Assets By Major Class [Line Items] |  |  |  |  |  |  |  |  |
Value of the asset |  |  |  |  |  | 16,700 | 16,700 |  | 16,600
Intangible Assets acquired (disposed) |  |  |  |  |  | 0 | 100 |  |
Insurance Licenses | Blue Waters |  |  |  |  |  |  |  |  |
Finite And Indefinite Lived Intangible Assets By Major Class [Line Items] |  |  |  |  |  |  |  |  |
Acquired licenses |  | 50 |  |  |  |  |  |  |
Insurance Licenses | AAIC, Aspen Specialty, AIUK [Member] |  |  |  |  |  |  |  |  |
Finite And Indefinite Lived Intangible Assets By Major Class [Line Items] |  |  |  |  |  |  |  |  |
Value of the asset |  |  |  |  |  | 16,600 |  |  |
Insurance Licenses | Aspen U.K. |  |  |  |  |  |  |  |  |
Finite And Indefinite Lived Intangible Assets By Major Class [Line Items] |  |  |  |  |  |  |  |  |
Acquired licenses |  |  |  |  |  | 2,100 |  |  |
Insurance Licenses | Aspen Specialty |  |  |  |  |  |  |  |  |
Finite And Indefinite Lived Intangible Assets By Major Class [Line Items] |  |  |  |  |  |  |  |  |
Acquired licenses |  |  |  |  |  | 4,500 |  |  |
Insurance Licenses | AAIC |  |  |  |  |  |  |  |  |
Finite And Indefinite Lived Intangible Assets By Major Class [Line Items] |  |  |  |  |  |  |  |  |
Acquired licenses |  |  |  |  |  | 10,000 |  |  |
Agency Relationships |  |  |  |  |  |  |  |  |
Finite And Indefinite Lived Intangible Assets By Major Class [Line Items] |  |  |  |  |  |  |  |  |
Value of the asset |  |  |  |  |  | $ 2,300 | 26,200 |  | 0
Intangible Assets acquired (disposed) | (21,800) |  |  |  |  |  | 28,100 |  |
Agency Relationships | AgriLogic |  |  |  |  |  |  |  |  |
Finite And Indefinite Lived Intangible Assets By Major Class [Line Items] |  |  |  |  |  |  |  |  |
Value of the asset |  |  |  | 25,000 |  |  |  |  |
Asset amortization period |  |  |  |  |  | 15 years |  |  |
Agency Relationships | Blue Waters |  |  |  |  |  |  |  |  |
Finite And Indefinite Lived Intangible Assets By Major Class [Line Items] |  |  |  |  |  |  |  |  |
Value of the asset |  | 3,100 |  |  |  |  |  |  |
Asset amortization period |  |  |  |  |  | 5 years |  |  |
Non-compete Agreements |  |  |  |  |  |  |  |  |
Finite And Indefinite Lived Intangible Assets By Major Class [Line Items] |  |  |  |  |  |  |  |  |
Value of the asset |  |  |  |  |  | $ 700 | 3,300 |  | 0
Intangible Assets acquired (disposed) | $ (900) |  |  |  |  |  | 3,900 |  |
Non-compete Agreements | AgriLogic |  |  |  |  |  |  |  |  |
Finite And Indefinite Lived Intangible Assets By Major Class [Line Items] |  |  |  |  |  |  |  |  |
Indefinite-lived and finite-lived intangible assets acquired |  |  |  | 2,900 |  |  |  |  |
Asset amortization period |  |  |  |  |  | 5 years |  |  |
Non-compete Agreements | Blue Waters |  |  |  |  |  |  |  |  |
Finite And Indefinite Lived Intangible Assets By Major Class [Line Items] |  |  |  |  |  |  |  |  |
Value of the asset |  | $ 1,000 |  |  |  |  |  |  |
Asset amortization period |  |  |  |  |  | 5 years |  |  |
Value of Business Acquired |  |  |  |  |  |  |  |  |
Finite And Indefinite Lived Intangible Assets By Major Class [Line Items] |  |  |  |  |  |  |  |  |
Value of the asset |  |  |  |  |  |  | 0 |  | 0
Intangible Assets acquired (disposed) |  |  |  |  |  |  | 1,800 |  |
Value of Business Acquired | AgriLogic |  |  |  |  |  |  |  |  |
Finite And Indefinite Lived Intangible Assets By Major Class [Line Items] |  |  |  |  |  |  |  |  |
Indefinite-lived and finite-lived intangible assets acquired |  |  |  | 1,800 |  |  |  |  |
Consulting Relationships |  |  |  |  |  |  |  |  |
Finite And Indefinite Lived Intangible Assets By Major Class [Line Items] |  |  |  |  |  |  |  |  |
Value of the asset |  |  |  |  |  | $ 0 | 900 |  | 0
Asset amortization period |  |  |  |  |  | 10 years |  |  |
Intangible Assets acquired (disposed) |  |  |  |  |  | $ 0 | 1,000 |  |
Consulting Relationships | AgriLogic |  |  |  |  |  |  |  |  |
Finite And Indefinite Lived Intangible Assets By Major Class [Line Items] |  |  |  |  |  |  |  |  |
Value of the asset |  |  |  | $ 1,000 |  |  |  |  |
Renewal Rights |  |  |  |  |  |  |  |  |
Finite And Indefinite Lived Intangible Assets By Major Class [Line Items] |  |  |  |  |  |  |  |  |
Value of the asset |  |  |  |  |  | 1,400 | 1,700 |  | $ 0
Intangible Assets acquired (disposed) |  |  |  |  |  | 0 | 1,900 |  |
AAIC, Aspen Specialty, AIUK [Member] | Insurance Licenses |  |  |  |  |  |  |  |  |
Finite And Indefinite Lived Intangible Assets By Major Class [Line Items] |  |  |  |  |  |  |  |  |
Value of the asset |  |  |  |  |  |  | 16,600 |  |
Liberty Specialty | Insurance Licenses |  |  |  |  |  |  |  |  |
Finite And Indefinite Lived Intangible Assets By Major Class [Line Items] |  |  |  |  |  |  |  |  |
Value of the asset |  |  | $ 1,900 |  |  |  |  |  |
Liberty Specialty | Renewal Rights |  |  |  |  |  |  |  |  |
Finite And Indefinite Lived Intangible Assets By Major Class [Line Items] |  |  |  |  |  |  |  |  |
Asset amortization period |  |  | 5 years |  |  |  |  |  |
Aspen U.K. |  |  |  |  |  |  |  |  |
Finite And Indefinite Lived Intangible Assets By Major Class [Line Items] |  |  |  |  |  |  |  |  |
Initial purchase price |  |  |  |  | $ 1,600 |  |  |  |
Aspen U.K. | Trade Mark |  |  |  |  |  |  |  |  |
Finite And Indefinite Lived Intangible Assets By Major Class [Line Items] |  |  |  |  |  |  |  |  |
Value of the asset |  |  |  |  |  |  | $ 1,600 |  |
Crop Re |  |  |  |  |  |  |  |  |
Finite And Indefinite Lived Intangible Assets By Major Class [Line Items] |  |  |  |  |  |  |  |  |
Goodwill, acquired during period |  |  |  |  |  | 0 |  |  |
Equity method investment, ownership percentage | 23.20% |  |  |  |  |  |  |  |
Digital Re |  |  |  |  |  |  |  |  |
Finite And Indefinite Lived Intangible Assets By Major Class [Line Items] |  |  |  |  |  |  |  |  |
Goodwill, acquired during period |  |  |  |  |  | $ 1,800 |  |  |
Equity method investment, ownership percentage |  |  |  |  |  |  |  | 49.00% |
AgriLogic | Disposal Group, Disposed of by Sale, Not Discontinued Operations [Member] |  |  |  |  |  |  |  |  |
Finite And Indefinite Lived Intangible Assets By Major Class [Line Items] |  |  |  |  |  |  |  |  |
Goodwill, disposed | $ (20,600) |  |  |  |  |  |  |  |
AgriLogic | Disposal Group, Disposed of by Sale, Not Discontinued Operations [Member] | Trade Mark |  |  |  |  |  |  |  |  |
Finite And Indefinite Lived Intangible Assets By Major Class [Line Items] |  |  |  |  |  |  |  |  |
Intangible Assets acquired (disposed) | (3,100) |  |  |  |  |  |  |  |
AgriLogic | Disposal Group, Disposed of by Sale, Not Discontinued Operations [Member] | Agency Relationships |  |  |  |  |  |  |  |  |
Finite And Indefinite Lived Intangible Assets By Major Class [Line Items] |  |  |  |  |  |  |  |  |
Intangible Assets acquired (disposed) | (21,800) |  |  |  |  |  |  |  |
AgriLogic | Disposal Group, Disposed of by Sale, Not Discontinued Operations [Member] | Non-compete Agreements |  |  |  |  |  |  |  |  |
Finite And Indefinite Lived Intangible Assets By Major Class [Line Items] |  |  |  |  |  |  |  |  |
Intangible Assets acquired (disposed) | $ (900) |  |  |  |  |  |  |  |

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- Definition Amount of consideration transferred, consisting of acquisition-date fair value of assets transferred by the acquirer, liabilities incurred by the acquirer, and equity interest issued by the acquirer. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 805 -SubTopic 30 -Section 30 -Paragraph 7 -URI http://asc.fasb.org/extlink&oid=6911251&loc=d3e6578-128477 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 805 -SubTopic 30 -Section 50 -Paragraph 1 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=35744584&loc=d3e6927-128479 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 805 -SubTopic 30 -Section 30 -Paragraph 8 -URI http://asc.fasb.org/extlink&oid=6911251&loc=d3e6613-128477 + Details
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- Definition Amount of liability recognized arising from contingent consideration in a business combination. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 805 -SubTopic 30 -Section 50 -Paragraph 1 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=35744584&loc=d3e6927-128479 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 805 -SubTopic 30 -Section 35 -Paragraph 1 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=6911338&loc=d3e6819-128478 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 805 -SubTopic 30 -Section 25 -Paragraph 6 -URI http://asc.fasb.org/extlink&oid=6911189&loc=d3e6408-128476 + Details
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- Definition The cash outflow associated with the acquisition of business during the period. The cash portion only of the acquisition price. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 13 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=98514028&loc=d3e3213-108585 + Details
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