Fair Value Of Financial Instruments (Schedule Of Fair Value Of Financial Instruments) (Details) - USD ($) $ in Millions		Jun. 30, 2016	Dec. 31, 2015
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading fixed maturity investments		$ 7,794.2	$ 7,201.5
Equity securities		202.3	403.0
Other invested assets		939.8	966.7
Total investments		8,936.3	8,571.2
Senior notes		1,293.7	1,292.9
Other long-term debt		23.1	23.0
Total Fair Value [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other invested assets		820.0	840.2
Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading fixed maturity investments		7,794.2	7,201.5
Equity securities		202.3	403.0
Other invested assets	[1]	820.0	840.2
Total investments		8,816.5	8,444.7
Senior notes		1,293.7	1,292.9
Other long-term debt		23.1	23.0
Fair Value, Measurements, Recurring [Member] | Foreign exchange contracts
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets		1.5	0.1
Derivative liabilities		0.4	3.0
Fair Value, Measurements, Recurring [Member] | Interest rate swaps
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative liabilities			0.5
Fair Value, Measurements, Recurring [Member] | U.S. Government And Government Agencies [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading fixed maturity investments		1,805.5	1,434.0
Fair Value, Measurements, Recurring [Member] | Non-U.S. Government And Government Agencies [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading fixed maturity investments		469.6	556.8
Fair Value, Measurements, Recurring [Member] | States, Municipalities And Political Subdivisions [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading fixed maturity investments		504.6	413.5
Fair Value, Measurements, Recurring [Member] | Corporate Debt, Financial Institutions [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading fixed maturity investments		1,223.6	1,275.4
Fair Value, Measurements, Recurring [Member] | Corporate Debt, Industrials [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading fixed maturity investments		1,476.8	1,308.1
Fair Value, Measurements, Recurring [Member] | Corporate Debt, Utilities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading fixed maturity investments		140.4	118.9
Fair Value, Measurements, Recurring [Member] | Agency mortgage-backed [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading fixed maturity investments		757.0	751.8
Fair Value, Measurements, Recurring [Member] | Non-agency residential mortgage-backed [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading fixed maturity investments		24.0	34.0
Fair Value, Measurements, Recurring [Member] | Commercial mortgage-backed [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading fixed maturity investments		604.2	582.8
Fair Value, Measurements, Recurring [Member] | Asset-backed [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading fixed maturity investments		788.5	726.2
Fair Value, Measurements, Recurring [Member] | Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading fixed maturity investments		1,765.2	1,396.4
Equity securities		202.3	403.0
Other invested assets	[1]	0.0	0.0
Total investments		1,967.5	1,799.4
Senior notes		0.0	0.0
Other long-term debt		0.0	0.0
Fair Value, Measurements, Recurring [Member] | Level 1 [Member] | Foreign exchange contracts
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets		0.0	0.0
Derivative liabilities		0.0	0.0
Fair Value, Measurements, Recurring [Member] | Level 1 [Member] | Interest rate swaps
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative liabilities			0.0
Fair Value, Measurements, Recurring [Member] | Level 1 [Member] | U.S. Government And Government Agencies [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading fixed maturity investments		1,765.2	1,396.4
Fair Value, Measurements, Recurring [Member] | Level 1 [Member] | Non-U.S. Government And Government Agencies [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading fixed maturity investments		0.0	0.0
Fair Value, Measurements, Recurring [Member] | Level 1 [Member] | States, Municipalities And Political Subdivisions [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading fixed maturity investments		0.0	0.0
Fair Value, Measurements, Recurring [Member] | Level 1 [Member] | Corporate Debt, Financial Institutions [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading fixed maturity investments		0.0	0.0
Fair Value, Measurements, Recurring [Member] | Level 1 [Member] | Corporate Debt, Industrials [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading fixed maturity investments		0.0	0.0
Fair Value, Measurements, Recurring [Member] | Level 1 [Member] | Corporate Debt, Utilities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading fixed maturity investments		0.0	0.0
Fair Value, Measurements, Recurring [Member] | Level 1 [Member] | Agency mortgage-backed [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading fixed maturity investments		0.0	0.0
Fair Value, Measurements, Recurring [Member] | Level 1 [Member] | Non-agency residential mortgage-backed [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading fixed maturity investments		0.0	0.0
Fair Value, Measurements, Recurring [Member] | Level 1 [Member] | Commercial mortgage-backed [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading fixed maturity investments		0.0	0.0
Fair Value, Measurements, Recurring [Member] | Level 1 [Member] | Asset-backed [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading fixed maturity investments		0.0	0.0
Fair Value, Measurements, Recurring [Member] | Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading fixed maturity investments		5,828.5	5,636.0
Equity securities		0.0	0.0
Other invested assets	[1]	0.0	0.0
Total investments		5,828.5	5,636.0
Senior notes		1,352.0	1,337.9
Other long-term debt		30.0	27.7
Fair Value, Measurements, Recurring [Member] | Level 2 [Member] | Foreign exchange contracts
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets		1.5	0.1
Derivative liabilities		0.4	3.0
Fair Value, Measurements, Recurring [Member] | Level 2 [Member] | Interest rate swaps
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative liabilities			0.5
Fair Value, Measurements, Recurring [Member] | Level 2 [Member] | U.S. Government And Government Agencies [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading fixed maturity investments		40.3	37.6
Fair Value, Measurements, Recurring [Member] | Level 2 [Member] | Non-U.S. Government And Government Agencies [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading fixed maturity investments		469.6	556.8
Fair Value, Measurements, Recurring [Member] | Level 2 [Member] | States, Municipalities And Political Subdivisions [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading fixed maturity investments		504.6	413.5
Fair Value, Measurements, Recurring [Member] | Level 2 [Member] | Corporate Debt, Financial Institutions [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading fixed maturity investments		1,223.6	1,275.4
Fair Value, Measurements, Recurring [Member] | Level 2 [Member] | Corporate Debt, Industrials [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading fixed maturity investments		1,476.8	1,308.1
Fair Value, Measurements, Recurring [Member] | Level 2 [Member] | Corporate Debt, Utilities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading fixed maturity investments		140.4	118.9
Fair Value, Measurements, Recurring [Member] | Level 2 [Member] | Agency mortgage-backed [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading fixed maturity investments		618.8	645.7
Fair Value, Measurements, Recurring [Member] | Level 2 [Member] | Non-agency residential mortgage-backed [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading fixed maturity investments		24.0	34.0
Fair Value, Measurements, Recurring [Member] | Level 2 [Member] | Commercial mortgage-backed [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading fixed maturity investments		604.2	582.8
Fair Value, Measurements, Recurring [Member] | Level 2 [Member] | Asset-backed [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading fixed maturity investments		726.2	663.2
Fair Value, Measurements, Recurring [Member] | Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading fixed maturity investments		200.5	169.1
Equity securities		0.0	0.0
Other invested assets	[1]	0.0	0.0
Total investments		200.5	169.1
Senior notes		0.0	0.0
Other long-term debt		0.0	0.0
Fair Value, Measurements, Recurring [Member] | Level 3 [Member] | Foreign exchange contracts
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets		0.0	0.0
Derivative liabilities		0.0	0.0
Fair Value, Measurements, Recurring [Member] | Level 3 [Member] | Interest rate swaps
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative liabilities			0.0
Fair Value, Measurements, Recurring [Member] | Level 3 [Member] | U.S. Government And Government Agencies [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading fixed maturity investments		0.0	0.0
Fair Value, Measurements, Recurring [Member] | Level 3 [Member] | Non-U.S. Government And Government Agencies [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading fixed maturity investments		0.0	0.0
Fair Value, Measurements, Recurring [Member] | Level 3 [Member] | States, Municipalities And Political Subdivisions [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading fixed maturity investments		0.0	0.0
Fair Value, Measurements, Recurring [Member] | Level 3 [Member] | Corporate Debt, Financial Institutions [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading fixed maturity investments		0.0	0.0
Fair Value, Measurements, Recurring [Member] | Level 3 [Member] | Corporate Debt, Industrials [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading fixed maturity investments		0.0	0.0
Fair Value, Measurements, Recurring [Member] | Level 3 [Member] | Corporate Debt, Utilities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading fixed maturity investments		0.0	0.0
Fair Value, Measurements, Recurring [Member] | Level 3 [Member] | Agency mortgage-backed [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading fixed maturity investments		138.2	106.1
Fair Value, Measurements, Recurring [Member] | Level 3 [Member] | Non-agency residential mortgage-backed [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading fixed maturity investments		0.0	0.0
Fair Value, Measurements, Recurring [Member] | Level 3 [Member] | Commercial mortgage-backed [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading fixed maturity investments		0.0	0.0
Fair Value, Measurements, Recurring [Member] | Level 3 [Member] | Asset-backed [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading fixed maturity investments		62.3	63.0
Fair Value, Measurements, Recurring [Member] | Total Fair Value [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading fixed maturity investments		7,794.2	7,201.5
Equity securities		202.3	403.0
Other invested assets	[1]	820.0	840.2
Total investments		8,816.5	8,444.7
Senior notes		1,352.0	1,337.9
Other long-term debt		30.0	27.7
Fair Value, Measurements, Recurring [Member] | Total Fair Value [Member] | Foreign exchange contracts
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets		1.5	0.1
Derivative liabilities		0.4	3.0
Fair Value, Measurements, Recurring [Member] | Total Fair Value [Member] | Interest rate swaps
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative liabilities			0.5
Fair Value, Measurements, Recurring [Member] | Total Fair Value [Member] | U.S. Government And Government Agencies [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading fixed maturity investments		1,805.5	1,434.0
Fair Value, Measurements, Recurring [Member] | Total Fair Value [Member] | Non-U.S. Government And Government Agencies [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading fixed maturity investments		469.6	556.8
Fair Value, Measurements, Recurring [Member] | Total Fair Value [Member] | States, Municipalities And Political Subdivisions [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading fixed maturity investments		504.6	413.5
Fair Value, Measurements, Recurring [Member] | Total Fair Value [Member] | Corporate Debt, Financial Institutions [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading fixed maturity investments		1,223.6	1,275.4
Fair Value, Measurements, Recurring [Member] | Total Fair Value [Member] | Corporate Debt, Industrials [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading fixed maturity investments		1,476.8	1,308.1
Fair Value, Measurements, Recurring [Member] | Total Fair Value [Member] | Corporate Debt, Utilities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading fixed maturity investments		140.4	118.9
Fair Value, Measurements, Recurring [Member] | Total Fair Value [Member] | Agency mortgage-backed [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading fixed maturity investments		757.0	751.8
Fair Value, Measurements, Recurring [Member] | Total Fair Value [Member] | Non-agency residential mortgage-backed [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading fixed maturity investments		24.0	34.0
Fair Value, Measurements, Recurring [Member] | Total Fair Value [Member] | Commercial mortgage-backed [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading fixed maturity investments		604.2	582.8
Fair Value, Measurements, Recurring [Member] | Total Fair Value [Member] | Asset-backed [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading fixed maturity investments		$ 788.5	$ 726.2

[1] In accordance with U.S. GAAP, other invested assets, excluding other private securities, are measured at fair value using the net asset value per share (or its equivalent) practical expedient and have not been classified in the fair value hierarchy.