Financial Derivatives (Narrative) (Details) - USD ($) $ in Millions |
9 Months Ended | 12 Months Ended |
|---|---|---|
Sep. 30, 2017 |
Dec. 31, 2016 |
|
| 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 | $ (1.3) | $ (2.5) |
| Total net up-front payments received | $ (3.0) | $ (3.3) |
| Minimum [Member] | ||
| Derivative [Line Items] | ||
| Low Range Of Spread On Written Credit Derivatives | 0.056% | 0.685% |
| Upfront points on written credit derivative in excess of threshold | 51.5 | 45.0 |
| Maximum [Member] | ||
| Derivative [Line Items] | ||
| Low Range Of Spread On Written Credit Derivatives | 12.958% | 6.366% |
| Upfront points on written credit derivative in excess of threshold | 93.5 | 72.6 |