Segment Information (Summary Of Segment Results) (Details) $ in Millions	3 Months Ended
	Mar. 31, 2017 USD ($) segments	Mar. 31, 2016 USD ($)
Segment Reporting Information [Line Items]
Gross premiums written	$ 860.9	$ 863.5
Net premiums written	676.1	704.0
Net premiums earned	544.9	580.1
Net losses and loss expenses	(359.0)	(372.4)
Acquisition costs	(77.1)	(88.3)
General and administrative expenses	(104.1)	(96.4)
Underwriting income	4.7	23.0
Other insurance-related income	1.3	0.6
Other insurance-related expenses	(2.3)	(1.1)
Segment income	3.7	22.5
Net investment income	52.3	53.3
Net realized investment gains	40.7	18.9
Amortization of intangible assets	(2.3)	(2.5)
Other expenses	(4.8)
Interest expense	(10.4)	(20.0)
Foreign exchange gain (loss)	(1.4)	3.0
Income before income taxes	$ 77.8	$ 75.2
Loss and loss expense ratio	65.90%	64.20%
Acquisition cost ratio	14.10%	15.20%
General and administrative expense ratio	19.10%	16.60%
Expense ratio	33.20%	31.80%
Combined ratio	99.10%	96.00%
Number of Operating Segments | segments	3
Operating segments | North American Insurance
Segment Reporting Information [Line Items]
Gross premiums written	$ 396.8	$ 379.2
Net premiums written	253.9	266.2
Net premiums earned	295.0	316.3
Net losses and loss expenses	(218.2)	(216.3)
Acquisition costs	(25.4)	(33.9)
General and administrative expenses	(59.0)	(52.2)
Underwriting income	(7.6)	13.9
Other insurance-related income	0.5	0.6
Other insurance-related expenses	(1.4)	(0.7)
Segment income	$ (8.5)	$ 13.8
Loss and loss expense ratio	74.00%	68.40%
Acquisition cost ratio	8.60%	10.70%
General and administrative expense ratio	20.00%	16.50%
Expense ratio	28.60%	27.20%
Combined ratio	102.60%	95.60%
Operating segments | Global Markets Insurance
Segment Reporting Information [Line Items]
Gross premiums written	$ 115.8	$ 115.5
Net premiums written	89.6	87.6
Net premiums earned	92.7	94.2
Net losses and loss expenses	(55.2)	(67.8)
Acquisition costs	(16.2)	(17.9)
General and administrative expenses	(28.2)	(29.0)
Underwriting income	(6.9)	(20.5)
Other insurance-related income	0.8	0.0
Other insurance-related expenses	(0.4)	0.0
Segment income	$ (6.5)	$ (20.5)
Loss and loss expense ratio	59.50%	72.00%
Acquisition cost ratio	17.50%	19.00%
General and administrative expense ratio	30.40%	30.80%
Expense ratio	47.90%	49.80%
Combined ratio	107.40%	121.80%
Operating segments | Reinsurance
Segment Reporting Information [Line Items]
Gross premiums written	$ 348.3	$ 368.8
Net premiums written	332.6	350.2
Net premiums earned	157.2	169.6
Net losses and loss expenses	(85.6)	(88.3)
Acquisition costs	(35.5)	(36.5)
General and administrative expenses	(16.9)	(15.2)
Underwriting income	19.2	29.6
Other insurance-related income	0.0	0.0
Other insurance-related expenses	(0.5)	(0.4)
Segment income	$ 18.7	$ 29.2
Loss and loss expense ratio	54.50%	52.10%
Acquisition cost ratio	22.60%	21.50%
General and administrative expense ratio	10.70%	8.90%
Expense ratio	33.30%	30.40%
Combined ratio	87.80%	82.50%