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Intangible Assets and Goodwill - Narrative (Details) - USD ($) $ in Thousands |  |  |  |  |  |  | 3 Months Ended | 9 Months Ended | 12 Months Ended |  |  |  |
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Dec. 18, 2017 | Jan. 01, 2017 | Oct. 31, 2016 | Sep. 22, 2016 | Jan. 19, 2016 | Apr. 05, 2005 | Sep. 30, 2018 | Sep. 30, 2017 | Sep. 30, 2018 | Sep. 30, 2017 | Dec. 31, 2017 | Sep. 18, 2018 | Jun. 30, 2018 | Jun. 30, 2017 | Dec. 31, 2016
Finite And Indefinite Lived Intangible Assets By Major Class [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Contingent consideration |  |  |  |  | $ 14,100 |  |  |  |  |  |  |  |  |  |
Consideration Transferred, Other |  |  |  |  | 22,800 |  |  |  |  |  |  |  |  |  |
Ceding commission |  |  |  |  | 2,000 |  |  |  |  |  |  |  |  |  |
Consideration transferred |  |  |  |  | 69,100 |  |  |  |  |  |  |  |  |  |
Software acquired |  |  |  |  | 12,000 |  |  |  |  |  |  |  |  |  |
Residual assets |  |  |  |  | 300 |  |  |  |  |  |  |  |  |  |
Total assets acquired, net |  |  |  |  | $ 47,000 |  |  |  |  |  |  |  |  |  |
Goodwill |  |  |  |  |  |  | $ 0 | $ 0 | $ 0 | $ 1,800 |  |  |  |  |
Impairment charge |  |  |  |  |  |  |  |  |  |  | $ 3,400 |  |  |  |
AgriLogic |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Finite And Indefinite Lived Intangible Assets By Major Class [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Percentage of interests acquired |  |  |  |  | 100.00% |  |  |  |  |  |  |  |  |  |
Aggregate consideration for sale and purchase agreement |  |  |  |  | $ 53,000 |  |  |  |  |  |  |  |  |  |
Goodwill |  |  |  |  | 22,100 |  |  |  |  |  |  |  |  |  |
Intangible assets, expected tax deductible amount |  |  |  |  | 34,000 |  |  |  |  |  |  |  |  |  |
Goodwill, expected tax deductible amount |  |  |  |  | $ 21,000 |  |  |  |  |  |  |  |  |  |
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% |  |  |  |  |  |  |  |  |  |  |  |
Consideration transferred |  |  | $ 8,000 |  |  |  |  |  |  |  |  |  |  |  |
Residual assets |  |  | 300 |  |  |  |  |  |  |  |  |  |  |  |
Total assets acquired, net |  |  | 5,750 |  |  |  |  |  |  |  |  |  |  |  |
Goodwill |  |  | 2,100 |  |  |  |  |  |  |  |  |  |  |  |
Agency Relationships |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Finite And Indefinite Lived Intangible Assets By Major Class [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Value of the asset |  |  |  |  | $ 25,000 |  | 1,900 | 24,600 | 1,900 | 24,600 | 2,300 |  | $ 2,000 | $ 25,100 | $ 26,200
Intangible Assets, Additions |  |  |  |  |  |  | 0 | 0 | $ 0 | 0 |  |  |  |  |
Asset amortization period |  |  |  |  |  |  |  |  | 15 years |  |  |  |  |  |
Trademark |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Finite And Indefinite Lived Intangible Assets By Major Class [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Value of the asset |  |  |  |  | 4,000 |  | 2,700 | 6,000 | $ 2,700 | 6,000 | 2,900 |  | 2,700 | 6,300 | 6,600
Intangible Assets, Additions |  |  |  |  |  |  | 0 | 0 | $ 0 | 0 |  |  |  |  |
Asset amortization period |  |  |  |  |  |  |  |  | 10 years |  |  |  |  |  |
Trademark | Blue Waters |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Finite And Indefinite Lived Intangible Assets By Major Class [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Value of the asset |  |  | 1,500 |  |  |  |  |  |  |  |  |  |  |  |
Asset amortization period |  |  |  |  |  |  |  |  | 5 years |  |  |  |  |  |
Insurance Licenses |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Finite And Indefinite Lived Intangible Assets By Major Class [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Value of the asset |  |  |  |  |  |  | 16,700 | 16,700 | $ 16,700 | 16,700 | 16,700 |  | 16,700 | 16,700 | 16,700
Intangible Assets, Additions |  |  |  |  |  |  | 0 | 0 | 0 | 0 |  |  |  |  |
Insurance Licenses | Blue Waters |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Finite And Indefinite Lived Intangible Assets By Major Class [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Value of trading licenses |  |  | 50 |  |  |  |  |  |  |  |  |  |  |  |
Contractual Rights |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Finite And Indefinite Lived Intangible Assets By Major Class [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Value of the asset |  |  |  |  |  |  | 1,100 | 1,400 | 1,100 | 1,400 | 1,400 |  | 1,200 | 1,500 | 1,700
Intangible Assets, Additions |  |  |  |  |  |  | 0 | 0 | 0 | 0 |  |  |  |  |
Consulting Relationships |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Finite And Indefinite Lived Intangible Assets By Major Class [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Value of the asset |  |  |  |  | 1,000 |  | 0 | 700 | 0 | 700 | 0 |  | 0 | 800 | 900
Intangible Assets, Additions |  |  |  |  |  |  | 0 | 0 | $ 0 | 0 |  |  |  |  |
Asset amortization period |  |  |  |  |  |  |  |  | 10 years |  |  |  |  |  |
Customer Lists | 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 |  |  |  |  |  |  | 400 | 2,700 | $ 400 | 2,700 | 700 |  | $ 500 | $ 3,000 | $ 3,300
Intangible Assets, Additions |  |  |  |  | 2,900 |  | 0 | $ 0 | $ 0 | $ 0 |  |  |  |  |
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 | AgriLogic |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Finite And Indefinite Lived Intangible Assets By Major Class [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Intangible Assets, Additions |  |  |  |  | $ 1,800 |  |  |  |  |  |  |  |  |  |
Crop Re |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Finite And Indefinite Lived Intangible Assets By Major Class [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Equity method investment, ownership percentage | 23.20% |  |  |  |  |  |  |  |  |  |  |  |  |  |
Digital Re |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Finite And Indefinite Lived Intangible Assets By Major Class [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Goodwill |  | $ 1,800 |  |  |  |  |  |  |  |  |  |  |  |  |
Equity method investment, ownership percentage |  | 49.00% |  |  |  |  |  |  |  |  |  |  |  |  |
LSML | Insurance Licenses |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Finite And Indefinite Lived Intangible Assets By Major Class [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Value of the asset |  |  |  | $ 1,900 |  |  |  |  |  |  |  |  |  |  |
LSML | Contractual 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] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Aggregate consideration for sale and purchase agreement |  |  |  |  |  | $ 1,600 |  |  |  |  |  |  |  |  |
Aspen U.K. | Trademark |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Finite And Indefinite Lived Intangible Assets By Major Class [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Value of the asset |  |  |  |  |  |  | 1,600 |  | $ 1,600 |  | 1,600 |  |  |  |
Aspen U.K. | Insurance Licenses |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Finite And Indefinite Lived Intangible Assets By Major Class [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Acquired licenses |  |  |  |  |  |  | 2,100 |  | 2,100 |  |  |  |  |  |
AAIC, Aspen Specialty, AIUK | Insurance Licenses |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Finite And Indefinite Lived Intangible Assets By Major Class [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Value of the asset |  |  |  |  |  |  | 16,600 |  | 16,600 |  | $ 16,600 |  |  |  |
AAIC | Insurance Licenses |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Finite And Indefinite Lived Intangible Assets By Major Class [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Acquired licenses |  |  |  |  |  |  | 10,000 |  | 10,000 |  |  |  |  |  |
Aspen Specialty | Insurance Licenses |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Finite And Indefinite Lived Intangible Assets By Major Class [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Acquired licenses |  |  |  |  |  |  | $ 4,500 |  | $ 4,500 |  |  |  |  |  |
Disposed of by Sale | AgriLogic |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Finite And Indefinite Lived Intangible Assets By Major Class [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Sale of goodwill | $ (20,600) |  |  |  |  |  |  |  |  |  |  |  |  |  |
Disposed of by Sale | AgriLogic | Trademark |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Finite And Indefinite Lived Intangible Assets By Major Class [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Intangible assets (disposed) | (3,100) |  |  |  |  |  |  |  |  |  |  |  |  |  |
Disposed of by Sale | AgriLogic | Customer Lists |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Finite And Indefinite Lived Intangible Assets By Major Class [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Intangible assets (disposed) | (21,800) |  |  |  |  |  |  |  |  |  |  |  |  |  |
Disposed of by Sale | AgriLogic | Non-compete Agreements |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Finite And Indefinite Lived Intangible Assets By Major Class [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Intangible assets (disposed) | $ (900) |  |  |  |  |  |  |  |  |  |  |  |  |  |
Disposed of by Sale | AgriLogic Consulting, LLC | Consulting Relationships |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Finite And Indefinite Lived Intangible Assets By Major Class [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Sale of interest of consulting business |  |  |  |  |  |  |  |  |  |  |  | 60.00% |  |  |

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