Segment Information (Summary Of Segment Results) (Details) $ in Millions	3 Months Ended
	Mar. 31, 2016 USD ($) segments	Mar. 31, 2015 USD ($)
Segment Reporting Information [Line Items]
Gross premiums written	$ 863.5	$ 880.6
Net premiums written	704.0	772.5
Net premiums earned	580.1	568.5
Net losses and loss expenses	(372.4)	(325.2)
Acquisition costs	(88.3)	(78.7)
General and administrative expenses	(96.4)	(97.1)
Underwriting income	23.0	67.5
Other insurance-related income	0.6	0.9
Other insurance-related expenses	(1.1)	(1.8)
Segment income	22.5	66.6
Net investment income	53.3	44.6
Net realized investment gains	18.9	45.0
Amortization of intangible assets	(2.5)	(0.6)
Interest expense	(20.0)	(14.4)
Foreign exchange gain (loss)	3.0	(9.9)
Income before income taxes	$ 75.2	$ 131.3
Loss and loss expense ratio	64.20%	57.20%
Acquisition cost ratio	15.20%	13.80%
General and administrative expense ratio	16.60%	17.10%
Expense ratio	31.80%	30.90%
Combined ratio	96.00%	88.10%
Number of Operating Segments | segments	3
Operating segments | North American Insurance
Segment Reporting Information [Line Items]
Gross premiums written	$ 379.2	$ 380.8
Net premiums written	266.2	296.9
Net premiums earned	316.3	313.0
Net losses and loss expenses	(216.3)	(195.5)
Acquisition costs	(33.9)	(31.0)
General and administrative expenses	(52.2)	(59.3)
Underwriting income	13.9	27.2
Other insurance-related income	0.6	0.9
Other insurance-related expenses	(0.7)	(0.8)
Segment income	$ 13.8	$ 27.3
Loss and loss expense ratio	68.40%	62.50%
Acquisition cost ratio	10.70%	9.90%
General and administrative expense ratio	16.50%	18.90%
Expense ratio	27.20%	28.80%
Combined ratio	95.60%	91.30%
Operating segments | Global Markets Insurance
Segment Reporting Information [Line Items]
Gross premiums written	$ 115.5	$ 60.6
Net premiums written	87.6	42.9
Net premiums earned	94.2	50.0
Net losses and loss expenses	(67.8)	(20.5)
Acquisition costs	(17.9)	(7.0)
General and administrative expenses	(29.0)	(18.0)
Underwriting income	(20.5)	4.5
Other insurance-related income	0.0	0.0
Other insurance-related expenses	0.0	(1.0)
Segment income	$ (20.5)	$ 3.5
Loss and loss expense ratio	72.00%	41.00%
Acquisition cost ratio	19.00%	14.00%
General and administrative expense ratio	30.80%	36.00%
Expense ratio	49.80%	50.00%
Combined ratio	121.80%	91.00%
Operating segments | Reinsurance
Segment Reporting Information [Line Items]
Gross premiums written	$ 368.8	$ 439.2
Net premiums written	350.2	432.7
Net premiums earned	169.6	205.5
Net losses and loss expenses	(88.3)	(109.2)
Acquisition costs	(36.5)	(40.7)
General and administrative expenses	(15.2)	(19.8)
Underwriting income	29.6	35.8
Other insurance-related income	0.0	0.0
Other insurance-related expenses	(0.4)	0.0
Segment income	$ 29.2	$ 35.8
Loss and loss expense ratio	52.10%	53.10%
Acquisition cost ratio	21.50%	19.80%
General and administrative expense ratio	8.90%	9.60%
Expense ratio	30.40%	29.40%
Combined ratio	82.50%	82.50%