Financial Derivatives (Narrative) (Details) - USD ($) $ in Millions |
6 Months Ended | 12 Months Ended |
|---|---|---|
Jun. 30, 2018 |
Dec. 31, 2017 |
|
| Derivative [Line Items] | ||
| Written credit derivative spread threshold | 20.00% | 20.00% |
| Credit Default Swap, Selling Protection [Member] | ||
| Derivative [Line Items] | ||
| Credit Risk Derivative in Excess of Threshold, at Fair Value, Net | $ (0.3) | $ (0.4) |
| Total net up-front payments received | $ (0.8) | $ (5.5) |
| Minimum [Member] | ||
| Derivative [Line Items] | ||
| Low Range Of Spread On Written Credit Derivatives | 0.364% | 0.154% |
| Upfront points on written credit derivative in excess of threshold | 51.9 | 51.4 |
| Maximum [Member] | ||
| Derivative [Line Items] | ||
| Low Range Of Spread On Written Credit Derivatives | 13.879% | 19.457% |
| Upfront points on written credit derivative in excess of threshold | 72.0 | 71.6 |