Goodwill and Other Intangible Assets (Narrative) (Details) - USD ($) $ in Thousands |
12 Months Ended | |||
|---|---|---|---|---|
Dec. 31, 2017 |
Dec. 31, 2016 |
Dec. 31, 2015 |
Mar. 02, 2015 |
|
| Minimum | ||||
| Goodwill and Intangible Assets [Line Items] | ||||
| Finite-lived intangible assets, useful life | 1 year | |||
| Maximum | ||||
| Goodwill and Intangible Assets [Line Items] | ||||
| Finite-lived intangible assets, useful life | 16 years | |||
| Weighted Average | ||||
| Goodwill and Intangible Assets [Line Items] | ||||
| Finite-lived intangible assets, useful life | 7 years 10 months 24 days | |||
| Investments in other ventures, under equity method | ||||
| Goodwill and Intangible Assets [Line Items] | ||||
| Goodwill | $ 7,818 | $ 7,818 | $ 7,818 | |
| Platinum Underwriters Holdings, Ltd. | ||||
| Goodwill and Intangible Assets [Line Items] | ||||
| Goodwill | $ 191,729 | |||
| Finite lived intangible assets acquired | 75,200 | |||
| Indefinite lived intangible assets acquired | $ 8,400 |