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Fair Value Measurements (Narrative) (Details) - USD ($) $ / shares in Units, $ in Thousands, shares in Millions |  | 3 Months Ended | 12 Months Ended |  |
--- | --- | --- | --- | --- | ---
Jul. 18, 2014 | Dec. 31, 2015 | Dec. 31, 2015 | Dec. 31, 2014 | Dec. 31, 2013 | Mar. 02, 2015 | Jun. 30, 2014
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |  |  |  |
Fair value transfers out of level 3 |  |  |  | $ 24,644 |  |  |
Other assets (liabilities) |  | $ (9,362) | $ (9,362) | (14,408) |  |  |
Long-term debt |  | 966,079 | 966,079 | 249,522 |  |  |
Senior Notes, fair value |  | 973,342 | 973,342 | 279,000 |  |  |
Net unrealized gains (losses) recognized in earnings |  |  | (13,549) | (1,393) | $ 75,789 |  |
Other Investments | Net investment income |  |  |  |  |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |  |  |  |
Net unrealized gains (losses) recognized in earnings |  |  | (13,500) | (1,400) | 75,800 |  |
Other Assets and Liabilities | Other income (loss) |  |  |  |  |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |  |  |  |
Net unrealized gains (losses) recognized in earnings |  |  | (400) | $ 0 | $ 0 |  |
Significant Unobservable Inputs (Level 3) |  |  |  |  |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |  |  |  |
Assets, fair value |  | 242,087 | 242,087 |  |  |  |
Convertible preferred stock | Trupanion |  |  |  |  |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |  |  |  |
Equity investment, fair value |  |  |  |  |  |  | $ 30,200
Common stock |  |  |  |  |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |  |  |  |
Equity investment, price per share |  |  |  |  |  | $ 102.47 |
Common stock | Trupanion |  |  |  |  |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |  |  |  |
Equity investment, fair value | $ 24,600 | $ 26,900 | $ 26,900 |  |  |  |
Equity investment, number of shares | 2.5 |  |  |  |  |  |
Equity investment, price per share | $ 10 |  |  |  |  |  |
U.S. treasuries |  |  |  |  |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |  |  |  |
Weighted average effective yield |  | 1.30% | 1.30% | 1.00% |  |  |
Municipal |  |  |  |  |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |  |  |  |
Weighted average effective yield |  | 2.00% | 2.00% |  |  |  |
Agencies |  |  |  |  |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |  |  |  |
Weighted average effective yield |  | 1.70% | 1.70% | 1.20% |  |  |
Non-U.S. government (Sovereign Debt) |  |  |  |  |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |  |  |  |
Weighted average effective yield |  | 1.40% | 1.40% | 1.10% |  |  |
Non-U.S. government-backed corporate |  |  |  |  |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |  |  |  |
Weighted average effective yield |  | 1.30% | 1.30% | 1.10% |  |  |
Corporate |  |  |  |  |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |  |  |  |
Weighted average effective yield |  | 3.80% | 3.80% | 3.20% |  |  |
Agency mortgage-backed |  |  |  |  |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |  |  |  |
Weighted average effective yield |  | 2.70% | 2.70% | 2.30% |  |  |
Weighted average life |  |  | 6 years 1 month 6 days | 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 |  | 3.80% | 3.80% | 3.40% |  |  |
Weighted average life |  |  | 4 years 3 months 18 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 |  | 4.70% | 4.70% | 4.30% |  |  |
Weighted average life |  |  | 5 years 4 months 24 days | 5 years |  |  |
Commercial mortgage-backed |  |  |  |  |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |  |  |  |
Weighted average effective yield |  | 2.90% | 2.90% | 2.10% |  |  |
Weighted average life |  |  | 3 years 8 months 12 days | 3 years 6 months |  |  |
Asset-backed |  |  |  |  |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |  |  |  |
Weighted average effective yield |  | 2.10% | 2.10% | 1.50% |  |  |
Weighted average life |  |  | 2 years 6 months | 2 years 6 months |  |  |
Private equity partnerships | Net Asset Valuation Technique | Significant Unobservable Inputs (Level 3) |  |  |  |  |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |  |  |  |
Assets, fair value |  | $ 214,800 | $ 214,800 |  |  |  |
Private equity partnerships | Minimum |  |  |  |  |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |  |  |  |
Liquidating period |  |  | 7 years |  |  |  |
Private equity partnerships | Maximum |  |  |  |  |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |  |  |  |
Liquidating period |  |  | 10 years |  |  |  |
Senior secured bank loan fund |  |  |  |  |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |  |  |  |
Investment in closed end fund |  | 23,200 | $ 23,200 |  |  |  |
Senior secured bank loan fund | Minimum |  |  |  |  |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |  |  |  |
Liquidating period |  |  | 4 years |  |  |  |
Senior secured bank loan fund | Maximum |  |  |  |  |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |  |  |  |
Liquidating period |  |  | 5 years |  |  |  |
Hedge funds |  |  |  |  |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |  |  |  |
Hedge fund side pocket investments |  | 2,300 | $ 2,300 |  |  |  |
Restricted Stock Units (RSUs) | Significant Other Observable Inputs (Level 2) |  |  |  |  |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |  |  |  |
Other assets (liabilities) |  | (12,300) | (12,300) |  |  |  |
Assumed and Ceded Re Insurance Contracts, Group Two [Member] | Internal Valuation Model Valuation Technique | Significant Unobservable Inputs (Level 3) |  |  |  |  |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |  |  |  |
Liabilities, fair value |  | 1,270 | $ 1,270 |  |  |  |
Assumed and Ceded Re Insurance Contracts, Group Two [Member] | Weighted Average | Internal Valuation Model Valuation Technique | Significant Unobservable Inputs (Level 3) |  |  |  |  |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |  |  |  |
Liquidity discount |  |  | 1.30% |  |  |  |
Fixed maturity investments |  |  |  |  |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |  |  |  |
Fair value transfers out of level 3 | $ 24,600 |  |  | $ 24,644 |  |  |
Fixed maturity investments | Significant Unobservable Inputs (Level 3) |  |  |  |  |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |  |  |  |
Assets, fair value |  | 7,618 | $ 7,618 |  |  |  |
Assumed and Ceded Re Insurance Contracts, Group Three [Member] | Internal Valuation Model Valuation Technique | Significant Unobservable Inputs (Level 3) |  |  |  |  |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |  |  |  |
Liabilities, fair value |  | 4,629 | 4,629 |  |  |  |
Corporate | Discounted Cash Flow Valuation Technique | Significant Unobservable Inputs (Level 3) |  |  |  |  |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |  |  |  |
Assets, fair value |  | 7,618 | $ 7,618 |  |  |  |
Corporate | Weighted Average | Discounted Cash Flow Valuation Technique | Significant Unobservable Inputs (Level 3) |  |  |  |  |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |  |  |  |
Dividend rate |  |  | 6.70% |  |  |  |
Liquidity discount |  |  | 1.00% |  |  |  |
Risk-free rate |  |  | 0.50% |  |  |  |
Credit spread |  |  | 2.90% |  |  |  |
Private equity partnerships | Net Asset Valuation Technique | Significant Unobservable Inputs (Level 3) |  |  |  |  |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |  |  |  |
Assets, fair value |  | $ 214,848 | $ 214,848 |  |  |  |
Private equity partnerships | Minimum | Net Asset Valuation Technique | Significant Unobservable Inputs (Level 3) |  |  |  |  |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |  |  |  |
Estimated performance |  | (24.60%) |  |  |  |  |
Private equity partnerships | Maximum | Net Asset Valuation Technique | Significant Unobservable Inputs (Level 3) |  |  |  |  |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |  |  |  |
Estimated performance |  | 39.00% |  |  |  |  |
Private equity partnerships | Weighted Average | Net Asset Valuation Technique | Significant Unobservable Inputs (Level 3) |  |  |  |  |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |  |  |  |
Estimated performance |  | 0.50% |  |  |  |  |
Other Investments |  |  |  |  |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |  |  |  |
Fair value transfers out of level 3 |  |  |  | $ 0 |  |  |
Other Investments | Significant Unobservable Inputs (Level 3) |  |  |  |  |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |  |  |  |
Assets, fair value |  | $ 240,368 | $ 240,368 |  |  |  |

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