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