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