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Fair Value Measurements (Narrative) (Details) - USD ($) $ in Thousands | 3 Months Ended | 6 Months Ended | 12 Months Ended
--- | --- | --- | ---
Jun. 30, 2015 | Jun. 30, 2014 | Jun. 30, 2015 | Jun. 30, 2014 | Dec. 31, 2014
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |  |
Other assets and (liabilities) | $ 67,036 |  | $ 67,036 |  | $ (13,908)
Debt | 972,362 |  | 972,362 |  | 249,522
Debt, fair value | 984,776 |  | 984,776 |  | 279,000
Net unrealized gains (losses) recognized in earnings |  |  | 3,238 | $ 8,879 |
Other Investments | Net investment income |  |  |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |  |
Net unrealized gains (losses) recognized in earnings | (1,600) | $ 3,900 | 3,200 | $ 8,900 |
Other assets and liabilities | Other income (loss) |  |  |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |  |
Net unrealized gains (losses) recognized in earnings | (300) |  | (300) |  |
Quoted Prices In Active Markets For Identical Assets (Level 1) |  |  |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |  |
Other assets and (liabilities) | (23) |  | (23) |  | (569)
Significant Other Observable Inputs (Level 2) |  |  |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |  |
Other assets and (liabilities) | (8,628) |  | (8,628) |  | (4,405)
Significant Unobservable Inputs (Level 3) |  |  |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |  |
Other assets and (liabilities) | 75,687 |  | 75,687 |  | $ (8,934)
Assets, fair value | 359,042 |  | 359,042 |  |
Significant Unobservable Inputs (Level 3) | Corporate | Discounted Cash Flow Valuation Technique |  |  |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |  |
Assets, fair value | 7,660 |  | $ 7,660 |  |
Significant Unobservable Inputs (Level 3) | Corporate | Weighted Average or Actual | Discounted Cash Flow Valuation Technique |  |  |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |  |
Dividend rate |  |  | 6.50% |  |
Liquidity discount |  |  | 1.00% |  |
Risk-free rate |  |  | 0.30% |  |
Credit spread |  |  | 3.60% |  |
Significant Unobservable Inputs (Level 3) | Private equity partnerships | Net Asset Valuation Technique |  |  |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |  |
Assets, fair value | $ 250,796 |  | $ 250,796 |  |
Significant Unobservable Inputs (Level 3) | Private equity partnerships | Weighted Average or Actual | Net Asset Valuation Technique |  |  |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |  |
Estimated performance | 3.10% |  |  |  |
Significant Unobservable Inputs (Level 3) | Private equity partnerships | Minimum | Net Asset Valuation Technique |  |  |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |  |
Estimated performance | (30.20%) |  |  |  |
Significant Unobservable Inputs (Level 3) | Private equity partnerships | Maximum | Net Asset Valuation Technique |  |  |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |  |
Estimated performance | 14.00% |  |  |  |
Significant Unobservable Inputs (Level 3) | Senior secured bank loan funds | Net Asset Valuation Technique |  |  |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |  |
Assets, fair value | $ 22,561 |  | 22,561 |  |
Significant Unobservable Inputs (Level 3) | Senior secured bank loan funds | Weighted Average or Actual | Net Asset Valuation Technique |  |  |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |  |
Estimated performance | 1.20% |  |  |  |
Significant Unobservable Inputs (Level 3) | Hedge funds | Net Asset Valuation Technique |  |  |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |  |
Assets, fair value | $ 2,338 |  | 2,338 |  |
Significant Unobservable Inputs (Level 3) | Hedge funds | Weighted Average or Actual | Net Asset Valuation Technique |  |  |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |  |
Estimated performance | 0.00% |  |  |  |
Significant Unobservable Inputs (Level 3) | Hedge funds | Minimum | Net Asset Valuation Technique |  |  |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |  |
Estimated performance | 0.00% |  |  |  |
Significant Unobservable Inputs (Level 3) | Hedge funds | Maximum | Net Asset Valuation Technique |  |  |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |  |
Estimated performance | 0.00% |  |  |  |
Significant Unobservable Inputs (Level 3) | Reinsurance deposit asset | Internal Valuation Model Valuation Technique |  |  |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |  |
Assets, fair value | $ 82,905 |  | $ 82,905 |  |
Significant Unobservable Inputs (Level 3) | Reinsurance deposit asset | Weighted Average or Actual | Internal Valuation Model Valuation Technique |  |  |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |  |
Estimated performance |  |  | 3.00% |  |
Significant Unobservable Inputs (Level 3) | Assumed reinsurance contract | Internal Valuation Model Valuation Technique |  |  |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |  |
Liabilities, fair value | (1,308) |  | $ (1,308) |  |
Significant Unobservable Inputs (Level 3) | Assumed and ceded (re)insurance contract | Internal Valuation Model Valuation Technique |  |  |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |  |
Liabilities, fair value | (5,896) |  | (5,896) |  |
Significant Unobservable Inputs (Level 3) | Weather contract | Internal Valuation Model Valuation Technique |  |  |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |  |
Liabilities, fair value | $ (14) |  | $ (14) |  |
U.S. treasuries |  |  |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |  |
Weighted average effective yield | 1.00% |  | 1.00% |  | 1.00%
Agencies |  |  |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |  |
Weighted average effective yield | 1.70% |  | 1.70% |  | 1.20%
Municipal |  |  |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |  |
Weighted average effective yield | 2.40% |  | 2.40% |  |
Non-U.S. government (Sovereign Debt) |  |  |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |  |
Weighted average effective yield | 1.20% |  | 1.20% |  | 1.10%
Non-U.S. government-backed corporate |  |  |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |  |
Weighted average effective yield | 1.10% |  | 1.10% |  | 1.10%
Corporate |  |  |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |  |
Weighted average effective yield | 3.20% |  | 3.20% |  | 3.20%
Agency mortgage-backed |  |  |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |  |
Weighted average effective yield | 2.20% |  | 2.20% |  | 2.30%
Weighted average life |  |  | 6 years |  | 5 years 7 months 6 days
Non-agency prime residential mortgage-backed |  |  |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |  |
Weighted average effective yield | 3.40% |  | 3.40% |  | 3.40%
Weighted average life |  |  | 4 years 1 month 6 days |  | 4 years 1 month 6 days
AltA non-agency mortgage-backed |  |  |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |  |
Weighted average effective yield | 4.10% |  | 4.10% |  | 4.30%
Weighted average life |  |  | 5 years 1 month 6 days |  | 5 years
Commercial mortgage-backed |  |  |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |  |
Weighted average effective yield | 2.40% |  | 2.40% |  | 2.10%
Weighted average life |  |  | 3 years |  | 3 years 6 months
Asset-backed |  |  |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |  |
Weighted average effective yield | 1.40% |  | 1.40% |  | 1.50%
Weighted average life |  |  | 2 years 4 months 24 days |  | 2 years 6 months
Restricted stock units (RSUs) | Significant Other Observable Inputs (Level 2) |  |  |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |  |
Other assets and (liabilities) | $ (4,600) |  | $ (4,600) |  |
Private equity partnerships | Minimum |  |  |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |  |
Liquidation period for fund assets |  |  | 7 years |  |
Private equity partnerships | Maximum |  |  |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |  |
Liquidation period for fund assets |  |  | 10 years |  |
Private equity partnerships | Significant Unobservable Inputs (Level 3) | Net Asset Valuation Technique |  |  |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |  |
Assets, fair value | 250,800 |  | $ 250,800 |  |
Senior secured bank loan funds |  |  |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |  |
Investment in closed end fund | 22,600 |  | $ 22,600 |  |
Senior secured bank loan funds | Minimum |  |  |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |  |
Liquidation period for fund assets |  |  | 4 years |  |
Senior secured bank loan funds | Maximum |  |  |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |  |
Liquidation period for fund assets |  |  | 5 years |  |
Hedge funds |  |  |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |  |
Hedge fund side pocket investments | $ 2,300 |  | $ 2,300 |  |

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