Financial Derivatives (Narrative) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Derivative [Line Items]
Written credit derivative spread threshold	20.00%	20.00%	20.00%
Upfront points on written credit derivative in excess of threshold		57.0
Credit Default Swap, Selling Protection [Member]
Derivative [Line Items]
Credit risk derivative in excess of threshold, at fair value, net	$ (0.1)	$ (0.1)	$ (1.0)
Total net up-front payments received	$ (2.0)	$ (3.3)	$ (2.0)
Minimum
Derivative [Line Items]
Spread on written credit derivatives	4.00%	1.09%	4.26%
Upfront points on written credit derivative in excess of threshold	56.2		36.9
Maximum
Derivative [Line Items]
Spread on written credit derivatives	27.48%	44.00%	81.51%
Upfront points on written credit derivative in excess of threshold	85.2		75.2