Segment Information (Summary Of Segment Results) (Details) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2016 USD ($)	Sep. 30, 2015 USD ($)	Sep. 30, 2016 USD ($) segments	Sep. 30, 2015 USD ($)
Segment Reporting Information [Line Items]
Gross premiums written	$ 730.2	$ 754.1	$ 2,394.1	$ 2,460.6
Net premiums written	528.3	607.0	1,810.5	1,983.2
Net premiums earned	584.0	650.7	1,767.1	1,865.6
Net losses and loss expenses	(376.3)	(416.9)	(1,114.1)	(1,173.6)
Acquisition costs	(82.0)	(100.1)	(257.7)	(279.4)
General and administrative expenses	(104.2)	(105.8)	(305.3)	(311.3)
Underwriting income (loss)	21.5	27.9	90.0	101.3
Other insurance-related income	1.8	0.7	7.6	2.5
Other insurance-related expenses	(2.2)	(1.3)	(5.0)	(4.3)
Segment income	21.1	27.3	92.6	99.5
Net investment income	50.6	45.7	159.7	133.0
Net realized investment gains (losses)	10.7	(113.6)	104.0	(88.8)
Amortization of intangible assets	(2.5)	(2.6)	(7.5)	(6.1)
Interest expense	(13.5)	(14.5)	(53.3)	(43.2)
Foreign exchange (loss) gain	(1.0)	0.8	4.9	(10.4)
Income (loss) before income taxes	$ 65.4	$ (56.9)	$ 300.4	$ 84.0
Loss and loss expense ratio	64.40%	64.10%	63.00%	62.90%
Acquisition cost ratio	14.00%	15.40%	14.60%	15.00%
General and administrative expense ratio	17.80%	16.30%	17.30%	16.70%
Expense ratio	31.80%	31.70%	31.90%	31.70%
Combined ratio	96.20%	95.80%	94.90%	94.60%
Number of operating segments | segments			3
Operating Segments [Member] | North American Insurance [Member]
Segment Reporting Information [Line Items]
Gross premiums written	$ 466.5	$ 460.4	$ 1,354.1	$ 1,358.6
Net premiums written	307.9	350.8	904.9	1,019.9
Net premiums earned	312.8	332.8	958.8	974.2
Net losses and loss expenses	(215.9)	(222.3)	(642.1)	(655.5)
Acquisition costs	(31.5)	(35.6)	(102.8)	(100.8)
General and administrative expenses	(55.1)	(58.3)	(164.1)	(175.7)
Underwriting income (loss)	10.3	16.6	49.8	42.2
Other insurance-related income	1.1	0.7	2.6	2.5
Other insurance-related expenses	(0.6)	(0.7)	(1.9)	(2.1)
Segment income	$ 10.8	$ 16.6	$ 50.5	$ 42.6
Loss and loss expense ratio	69.00%	66.80%	67.00%	67.30%
Acquisition cost ratio	10.10%	10.70%	10.70%	10.30%
General and administrative expense ratio	17.60%	17.50%	17.10%	18.00%
Expense ratio	27.70%	28.20%	27.80%	28.30%
Combined ratio	96.70%	95.00%	94.80%	95.60%
Operating Segments [Member] | Global Markets Insurance [Member]
Segment Reporting Information [Line Items]
Gross premiums written	$ 126.7	$ 132.7	$ 367.6	$ 328.2
Net premiums written	91.3	104.1	267.1	216.2
Net premiums earned	96.8	110.3	285.1	263.0
Net losses and loss expenses	(74.5)	(70.9)	(203.4)	(157.2)
Acquisition costs	(16.4)	(22.7)	(50.6)	(54.1)
General and administrative expenses	(32.6)	(29.3)	(93.2)	(78.1)
Underwriting income (loss)	(26.7)	(12.6)	(62.1)	(26.4)
Other insurance-related income	0.7	0.0	0.7	0.0
Other insurance-related expenses	(0.3)	(0.6)	(0.3)	(2.2)
Segment income	$ (26.3)	$ (13.2)	$ (61.7)	$ (28.6)
Loss and loss expense ratio	77.00%	64.30%	71.30%	59.80%
Acquisition cost ratio	17.00%	20.60%	17.70%	20.60%
General and administrative expense ratio	33.70%	26.60%	32.70%	29.70%
Expense ratio	50.70%	47.20%	50.40%	50.30%
Combined ratio	127.70%	111.50%	121.70%	110.10%
Operating Segments [Member] | Reinsurance [Member]
Segment Reporting Information [Line Items]
Gross premiums written	$ 137.0	$ 161.0	$ 672.4	$ 773.8
Net premiums written	129.1	152.1	638.5	747.1
Net premiums earned	174.4	207.6	523.2	628.4
Net losses and loss expenses	(85.9)	(123.7)	(268.6)	(360.9)
Acquisition costs	(34.1)	(41.8)	(104.3)	(124.5)
General and administrative expenses	(16.5)	(18.2)	(48.0)	(57.5)
Underwriting income (loss)	37.9	23.9	102.3	85.5
Other insurance-related income	0.0	0.0	4.3	0.0
Other insurance-related expenses	(1.3)	0.0	(2.8)	0.0
Segment income	$ 36.6	$ 23.9	$ 103.8	$ 85.5
Loss and loss expense ratio	49.30%	59.60%	51.30%	57.40%
Acquisition cost ratio	19.50%	20.10%	19.90%	19.80%
General and administrative expense ratio	9.50%	8.70%	9.20%	9.10%
Expense ratio	29.00%	28.80%	29.10%	28.90%
Combined ratio	78.30%	88.40%	80.40%	86.30%