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Fair Value Measurements (Tables) | 12 Months Ended
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Dec. 31, 2017
Fair Value Disclosures [Abstract] |
Financial Assets Measured on Recurring Basis | The following tables present the level within the fair value hierarchy at which the Company’s financial assets and liabilities are measured on a recurring basis as at December 31, 2017 and December 31, 2016:
|  | As at December 31, 2017
|  | Level 1 |  | Level 2 |  | Level 3 |  | Total
|  | ($ in millions)
Available for sale financial assets, at fair value |  |  |  |  |  |  |  |
U.S. government |  | $ | 1,159.4 |  |  | $ | — |  |  | $ | — |  |  | $ | 1,159.4 |
U.S. agency |  | — |  |  | 52.1 |  |  | — |  |  | 52.1 |
Municipal |  | — |  |  | 54.9 |  |  | — |  |  | 54.9 |
Corporate |  | — |  |  | 2,415.7 |  |  | — |  |  | 2,415.7 |
Non-U.S. government-backed corporate |  | — |  |  | 91.3 |  |  | — |  |  | 91.3 |
Non-U.S. government |  | 341.2 |  |  | 143.7 |  |  | — |  |  | 484.9 |
Asset-backed |  | — |  |  | 26.2 |  |  | — |  |  | 26.2 |
Agency mortgage-backed |  | — |  |  | 946.5 |  |  | — |  |  | 946.5 |
Total fixed income securities available for sale, at fair value |  | 1,500.6 |  |  | 3,730.4 |  |  | — |  |  | 5,231.0 |
Short-term investments available for sale, at fair value |  | 87.3 |  |  | 2.6 |  |  | — |  |  | 89.9 |
Held for trading financial assets, at fair value |  |  |  |  |  |  |  |
U.S. government |  | 161.9 |  |  | — |  |  | — |  |  | 161.9 |
Municipal |  | — |  |  | 32.2 |  |  | — |  |  | 32.2 |
Corporate |  | — |  |  | 1,046.3 |  |  | — |  |  | 1,046.3 |
Non-U.S. government-backed corporate |  | — |  |  | 1.0 |  |  | — |  |  | 1.0 |
Non-U.S. government |  | 24.5 |  |  | 178.0 |  |  | — |  |  | 202.5 |
Asset-backed |  | — |  |  | 9.9 |  |  | — |  |  | 9.9 |
Agency Mortgage-Backed |  | — |  |  | 195.5 |  |  | — |  |  | 195.5 |
Total fixed income securities trading, at fair value |  | 186.4 |  |  | 1,462.9 |  |  | — |  |  | 1,649.3 |
Short-term investments trading, at fair value |  | 73.0 |  |  | — |  |  | — |  |  | 73.0 |
Equity investments trading, at fair value |  | 491.0 |  |  | — |  |  | — |  |  | 491.0 |
Catastrophe bonds trading, at fair value |  | — |  |  | 32.4 |  |  | — |  |  | 32.4 |
Other financial assets and liabilities, at fair value |  |  |  |  |  |  |  |
Derivatives at fair value — foreign exchange contracts |  | — |  |  | 6.4 |  |  | — |  |  | 6.4 |
Liabilities under derivative contracts — foreign exchange contracts |  | — |  |  | (1.0 | ) |  | — |  |  | (1.0 | )
Loan notes issued by variable interest entities, at fair value |  | — |  |  | — |  |  | (44.2 | ) |  | (44.2 | )
Loan notes issued by variable interest entities, at fair value (included within accrued expenses and other payables) |  | — |  |  | — |  |  | (42.4 | ) |  | (42.4 | )
Total |  | $ | 2,338.3 |  |  | $ | 5,233.7 |  |  | $ | (86.6 | ) |  | $ | 7,485.4 |
Transfers of assets into or out of a particular level are recorded at their fair values as of the end of each reporting period, consistent with the date of the determination of fair value. During the twelve months ended December 31, 2017 there were no transfers between Level 1, 2 and 3. The Company settled $115.6 million Level 3 liabilities in respect of the loan notes issued by the Silverton VIE for the twelve months ended December 31, 2017. As at December 31, 2017, there were no assets classified as Level 3 and the Company’s Level 3 liabilities consisted of the loan notes issued by the Silverton VIE.
| At December 31, 2016
| Level 1 |  | Level 2 |  | Level 3 |  | Total
| ($ in millions)
Available for sale financial assets, at fair value |  |  |  |  |  |  |
U.S. government | $ | 1,206.1 |  |  | $ | — |  |  | $ | — |  |  | $ | 1,206.1 |
U.S. agency | — |  |  | 119.6 |  |  | — |  |  | 119.6 |
Municipal | — |  |  | 24.4 |  |  | — |  |  | 24.4 |
Corporate | — |  |  | 2,586.5 |  |  | — |  |  | 2,586.5 |
Non-U.S. government-backed corporate | — |  |  | 89.8 |  |  | — |  |  | 89.8 |
Non-U.S. government | 343.7 |  |  | 145.0 |  |  | — |  |  | 488.7 |
Asset-backed | — |  |  | 63.0 |  |  | — |  |  | 63.0 |
Non-agency commercial mortgage-backed | — |  |  | 12.6 |  |  | — |  |  | 12.6 |
Agency mortgage-backed | — |  |  | 1,073.9 |  |  | — |  |  | 1,073.9 |
Total fixed income securities available for sale, at fair value | 1,549.8 |  |  | 4,114.8 |  |  | — |  |  | 5,664.6 |
Short-term investments available for sale, at fair value | 118.6 |  |  | 26.7 |  |  | — |  |  | 145.3 |
Held for trading financial assets, at fair value |  |  |  |  |  |  |
U.S. government | 82.4 |  |  | — |  |  | — |  |  | 82.4 |
Municipal | — |  |  | 15.5 |  |  | — |  |  | 15.5 |
Corporate | — |  |  | 820.6 |  |  | — |  |  | 820.6 |
Non-U.S. government | — |  |  | 202.8 |  |  | — |  |  | 202.8 |
Asset-backed | — |  |  | 14.5 |  |  | — |  |  | 14.5 |
Agency mortgage-backed | — |  |  | 129.9 |  |  | — |  |  | 129.9 |
Total fixed income securities trading, at fair value | 82.4 |  |  | 1,183.3 |  |  | — |  |  | 1,265.7 |
Short-term investments trading, at fair value | 76.1 |  |  | 109.3 |  |  | — |  |  | 185.4 |
Equity investments trading, at fair value | 584.7 |  |  | — |  |  | — |  |  | 584.7 |
Catastrophe bonds trading, at fair value | — |  |  | 42.5 |  |  | — |  |  | 42.5 |
Other financial assets and liabilities, at fair value |  |  |  |  |  |  |
Derivatives at fair value — foreign exchange contracts | — |  |  | 7.2 |  |  | — |  |  | 7.2 |
Liabilities under derivative contracts — foreign exchange contracts | — |  |  | (18.4 | ) |  | — |  |  | (18.4 | )
Loan notes issued by variable interest entities, at fair value | — |  |  | — |  |  | (115.0 | ) |  | (115.0 | )
Loan notes issued by variable interest entities, at fair value (included within accrued expenses and other payables) | — |  |  | — |  |  | (108.4 | ) |  | (108.4 | )
Total | $ | 2,411.6 |  |  | $ | 5,465.4 |  |  | $ | (223.4 | ) |  | $ | 7,653.6 |
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation | The following table presents a reconciliation of the beginning and ending balances for all assets and liabilities measured at fair value on a recurring basis using Level 3 inputs for the twelve months ended December 31, 2017:
Reconciliation of Liabilities Using Level 3 Inputs |  |  | Twelve Months Ended December 31, 2017 |  | Twelve Months Ended December 31, 2016
|  |  | ($ in millions)
Balance at the beginning of the period (1) |  | $ | 223.4 |  |  | $ | 190.6 |
Distributed to third party |  | (115.6 | ) |  | (89.3 | )
Loan notes issued during the period |  | — |  |  | 105.0 |
Total change in fair value included in the statement of operations |  | (21.2 | ) |  | 17.1 |
Balance at the end of the period (1) |  | $ | 86.6 |  |  | $ | 223.4 |
______________
(1) | The amount classified as other payables was $42.4 million and $108.4 million as at December 31, 2017 and December 31, 2016, respectively.
Pricing Sources Used in Pricing Fixed Income Investments | Pricing sources used in pricing fixed income investments as at December 31, 2017 and December 31, 2016 were as follows:
|  | As at December 31, 2017 |  | At December 31, 2016
Index providers |  | 84 | % |  | 87 | %
Pricing services |  | 11 |  |  | 7 |
Broker-dealers |  | 5 |  |  | 6 |
Total |  | 100 | % |  | 100 | %
Summary of Securities Priced Using Pricing Information from Index Providers | A summary of securities priced using pricing information from index providers as at December 31, 2017 and December 31, 2016 is provided below:
|  | As at December 31, 2017 |  | At December 31, 2016
|  | Fair Market Value Determined using Prices from Index Providers |  | % of Total Fair Value from Index Providers |  | Fair Market Value Determined using Prices from Index Providers |  | % of Total Fair Value from Index Providers
|  | ($ in millions, except for percentages)
U.S. government |  | $ | 1,321.3 |  |  | 100% |  | $ | 1,288.2 |  |  | 100%
U.S. agency |  | 43.4 |  |  | 83% |  | 110.2 |  |  | 92%
Municipal |  | 37.4 |  |  | 43% |  | 28.8 |  |  | 72%
Corporate |  | 3,299.6 |  |  | 83% |  | 3,275.3 |  |  | 96%
Non-U.S. government-backed corporate |  | 44.0 |  |  | 48% |  | 44.8 |  |  | 50%
Non-U.S. government |  | 399.4 |  |  | 58% |  | 455.6 |  |  | 72%
Asset-backed |  | 13.5 |  |  | 37% |  | 32.1 |  |  | 41%
Non-agency commercial mortgage-backed |  | — |  |  | —% |  | 12.5 |  |  | 98%
Agency mortgage-backed |  | 605.0 |  |  | 53% |  | 691.9 |  |  | 58%
Total fixed income securities |  | $ | 5,763.6 |  |  | 84% |  | $ | 5,939.4 |  |  | 87%
Equities |  | $ | 491.0 |  |  | 100% |  | $ | 584.7 |  |  | 100%
Total fixed income securities and equity investments |  | $ | 6,254.6 |  |  | 85% |  | $ | 6,524.1 |  |  | 88%
Fair Value Inputs, Assets, Quantitative Information | The observable and unobservable inputs used to determine the fair value of the Loan Notes as at December 31, 2017 and December 31, 2016, respectively, are presented in the tables below:
At December 31, 2017 |  | Fair Value Level 3 |  | Valuation Method |  | Observable (O) and Unobservable (U) inputs |  | Low |  | High
| ($ in millions) |  |  |  |  | ($ in millions)
Loan Notes |  | $ | 86.6 |  | (1) | Internal Valuation Model |  | Gross premiums written (O) |  | $ | 50.1 |  |  | $ | 61.1 |
|  |  |  |  |  | Reserve for losses (U) |  | $ | 4.2 |  |  | $ | 61.9 |
|  |  |  |  |  | Contract period (O) |  | N/A |  |  | 365 days |
|  |  |  |  |  | Initial value of issuance (O) |  | $ | 325.0 |  |  | $ | 325.0 |
At December 31, 2016 |  | Fair Value Level 3 |  | Valuation Method |  | Observable (O) and Unobservable (U) inputs |  | Low |  | High
| ($ in millions) |  |  |  |  | ($ in millions)
Loan Notes |  | $ | 223.4 |  | (1) | Internal Valuation Model |  | Gross premiums written (O) |  | $ | 38.9 |  |  | $ | 43.4 |
|  |  |  |  |  | Reserve for losses (U) |  | $ | 2.7 |  |  | $ | 11.8 |
|  |  |  |  |  | Contract period (O) |  | N/A |  |  | 365 days |
|  |  |  |  |  | Initial value of issuance (O) |  | $ | 220.0 |  |  | $ | 220.0 |
______________
(1) | The amount classified as other payables was $42.4 million and $108.4 million as at December 31, 2017 and December 31, 2016, respectively.

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- Definition Tabular disclosure of assets, including [financial] instruments measured at fair value that are classified in stockholders' equity, if any, by class that are measured at fair value on a recurring basis. The disclosures contemplated herein include the fair value measurements at the reporting date by the level within the fair value hierarchy in which the fair value measurements in their entirety fall, segregating fair value measurements using quoted prices in active markets for identical assets (Level 1), significant other observable inputs (Level 2), and significant unobservable inputs (Level 3). + 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 (b) -URI http://asc.fasb.org/extlink&oid=66048111&loc=d3e19207-110258 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=66048111&loc=d3e19190-110258 Reference 3: 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=66048111&loc=d3e19207-110258 + Details
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- Definition Tabular disclosure of quantitative information about the inputs used in the fair value measurement of assets. This disclosure may include, but is not limited to, the fair value of the asset, valuation technique used to measure fair value, the inputs used to measure fair value, the ranges of the inputs, and the weighted averages of the inputs. + 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 (bbb) -URI http://asc.fasb.org/extlink&oid=66048111&loc=d3e19207-110258 + Details
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- Definition Tabular disclosure of the fair value measurement of liabilities using significant unobservable inputs (Level 3), a reconciliation of the beginning and ending balances, separately presenting changes attributable to the following: (1) total gains or losses for the period (realized and unrealized), segregating those gains or losses included in earnings (or changes in net assets), and gains or losses recognized in other comprehensive income (loss) and a description of where those gains or losses included in earnings (or changes in net assets) are reported in the statement of income (or activities); (2) purchases, sales, issues, and settlements (each type disclosed separately); and (3) transfers in and transfers out of Level 3 (for example, transfers due to changes in the observability of significant inputs) by class of liability. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 3 -URI http://asc.fasb.org/extlink&oid=66048111&loc=d3e19279-110258 Reference 2: 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=66048111&loc=d3e19207-110258 + Details
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