Fair Value Of Financial Instruments (Schedule Of Fair Value Of Financial Instruments) (Details) - USD ($) $ in Millions		3 Months Ended
		Mar. 31, 2017	Dec. 31, 2016
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading fixed maturity investments		$ 6,473.1	$ 6,737.7
Equity securities		255.2	243.9
Other invested assets		966.6	960.7
Total investments		7,694.9	7,942.3
Senior notes		794.4	794.2
Other long-term debt		$ 22.6	22.0
Fair value inputs, discount rate		10.00%
Total Fair Value [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other invested assets		$ 902.7	897.8
Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading fixed maturity investments		6,473.1	6,737.7
Equity securities		255.2	243.9
Other invested assets	[1]	902.7	897.8
Total investments		7,631.0	7,879.4
Senior notes		794.4	794.2
Other long-term debt		22.6	22.0
Fair Value, Measurements, Recurring [Member] | Foreign exchange contracts
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets			10.4
Derivative liabilities		0.3	0.1
Fair Value, Measurements, Recurring [Member] | Interest rate swaps
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative liabilities		0.2
Fair Value, Measurements, Recurring [Member] | Insurance contracts
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets		6.5	7.4
Fair Value, Measurements, Recurring [Member] | Reinsurance contracts
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative liabilities		3.4	4.0
Fair Value, Measurements, Recurring [Member] | U.S. government and government agencies
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading fixed maturity investments		1,410.5	1,426.0
Fair Value, Measurements, Recurring [Member] | Non-U.S. government and government agencies
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading fixed maturity investments		543.2	469.9
Fair Value, Measurements, Recurring [Member] | States, municipalities and political subdivisions
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading fixed maturity investments		41.7	354.1
Fair Value, Measurements, Recurring [Member] | Corporate Debt: Financial institutions
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading fixed maturity investments		1,046.4	1,032.7
Fair Value, Measurements, Recurring [Member] | Corporate debt: Industrials
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading fixed maturity investments		1,379.2	1,321.3
Fair Value, Measurements, Recurring [Member] | Corporate debt: Utilities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading fixed maturity investments		153.2	140.0
Fair Value, Measurements, Recurring [Member] | Agency mortgage-backed
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading fixed maturity investments		595.6	614.5
Fair Value, Measurements, Recurring [Member] | Non-agency residential mortgage-backed
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading fixed maturity investments		23.4	23.9
Fair Value, Measurements, Recurring [Member] | Commercial mortgage-backed
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading fixed maturity investments		467.1	598.0
Fair Value, Measurements, Recurring [Member] | Asset-backed
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading fixed maturity investments		812.8	757.3
Fair Value, Measurements, Recurring [Member] | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading fixed maturity investments		1,357.5	1,379.6
Equity securities		230.7	219.2
Other invested assets	[1]	0.0	0.0
Total investments		1,588.2	1,598.8
Senior notes		0.0	0.0
Other long-term debt		0.0	0.0
Fair Value, Measurements, Recurring [Member] | Level 1 | Foreign exchange contracts
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets			0.0
Derivative liabilities		0.0	0.0
Fair Value, Measurements, Recurring [Member] | Level 1 | 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 | Insurance contracts
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets		0.0	0.0
Fair Value, Measurements, Recurring [Member] | Level 1 | Reinsurance contracts
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative liabilities		0.0	0.0
Fair Value, Measurements, Recurring [Member] | Level 1 | U.S. government and government agencies
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading fixed maturity investments		1,357.5	1,379.6
Fair Value, Measurements, Recurring [Member] | Level 1 | Non-U.S. government and government agencies
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 | States, municipalities and political subdivisions
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 | Corporate Debt: Financial institutions
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 | Corporate debt: Industrials
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 | Corporate debt: Utilities
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 | Agency mortgage-backed
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 | Non-agency residential mortgage-backed
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 | Commercial mortgage-backed
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 | Asset-backed
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
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading fixed maturity investments		4,912.3	5,158.5
Equity securities		0.0	0.0
Other invested assets	[1]	0.0	0.0
Total investments		4,912.3	5,158.5
Senior notes		825.6	827.1
Other long-term debt		27.1	26.7
Fair Value, Measurements, Recurring [Member] | Level 2 | Foreign exchange contracts
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets			10.4
Derivative liabilities		0.3	0.1
Fair Value, Measurements, Recurring [Member] | Level 2 | Interest rate swaps
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative liabilities		0.2
Fair Value, Measurements, Recurring [Member] | Level 2 | Insurance contracts
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets		0.0	0.0
Fair Value, Measurements, Recurring [Member] | Level 2 | Reinsurance contracts
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative liabilities		0.0	0.0
Fair Value, Measurements, Recurring [Member] | Level 2 | U.S. government and government agencies
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading fixed maturity investments		53.0	46.4
Fair Value, Measurements, Recurring [Member] | Level 2 | Non-U.S. government and government agencies
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading fixed maturity investments		543.2	469.9
Fair Value, Measurements, Recurring [Member] | Level 2 | States, municipalities and political subdivisions
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading fixed maturity investments		41.7	354.1
Fair Value, Measurements, Recurring [Member] | Level 2 | Corporate Debt: Financial institutions
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading fixed maturity investments		1,044.4	1,031.2
Fair Value, Measurements, Recurring [Member] | Level 2 | Corporate debt: Industrials
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading fixed maturity investments		1,379.2	1,321.3
Fair Value, Measurements, Recurring [Member] | Level 2 | Corporate debt: Utilities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading fixed maturity investments		153.2	140.0
Fair Value, Measurements, Recurring [Member] | Level 2 | Agency mortgage-backed
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading fixed maturity investments		444.3	475.9
Fair Value, Measurements, Recurring [Member] | Level 2 | Non-agency residential mortgage-backed
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading fixed maturity investments		23.4	23.9
Fair Value, Measurements, Recurring [Member] | Level 2 | Commercial mortgage-backed
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading fixed maturity investments		467.1	598.0
Fair Value, Measurements, Recurring [Member] | Level 2 | Asset-backed
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading fixed maturity investments		762.8	697.8
Fair Value, Measurements, Recurring [Member] | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading fixed maturity investments		203.3	199.7
Equity securities		24.5	24.7
Other invested assets	[1]	0.0	0.0
Total investments		227.8	224.4
Senior notes		0.0	0.0
Other long-term debt		0.0	0.0
Fair Value, Measurements, Recurring [Member] | Level 3 | Foreign exchange contracts
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets			0.0
Derivative liabilities		0.0	0.0
Fair Value, Measurements, Recurring [Member] | Level 3 | 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 | Insurance contracts
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets		6.5	7.4
Fair Value, Measurements, Recurring [Member] | Level 3 | Reinsurance contracts
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative liabilities		3.4	4.0
Fair Value, Measurements, Recurring [Member] | Level 3 | U.S. government and government agencies
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 | Non-U.S. government and government agencies
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 | States, municipalities and political subdivisions
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 | Corporate Debt: Financial institutions
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading fixed maturity investments		2.0	1.5
Fair Value, Measurements, Recurring [Member] | Level 3 | Corporate debt: Industrials
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 | Corporate debt: Utilities
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 | Agency mortgage-backed
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading fixed maturity investments		151.3	138.7
Fair Value, Measurements, Recurring [Member] | Level 3 | Non-agency residential mortgage-backed
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 | Commercial mortgage-backed
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 | Asset-backed
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading fixed maturity investments		50.0	59.5
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		6,473.1	6,737.7
Equity securities		255.2	243.9
Other invested assets	[1]	902.7	897.8
Total investments		7,631.0	7,879.4
Senior notes		825.6	827.1
Other long-term debt		27.1	26.7
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			10.4
Derivative liabilities		0.3	0.1
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		0.2
Fair Value, Measurements, Recurring [Member] | Total Fair Value [Member] | Insurance contracts
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets		6.5	7.4
Fair Value, Measurements, Recurring [Member] | Total Fair Value [Member] | Reinsurance contracts
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative liabilities		3.4	4.0
Fair Value, Measurements, Recurring [Member] | Total Fair Value [Member] | U.S. government and government agencies
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading fixed maturity investments		1,410.5	1,426.0
Fair Value, Measurements, Recurring [Member] | Total Fair Value [Member] | Non-U.S. government and government agencies
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading fixed maturity investments		543.2	469.9
Fair Value, Measurements, Recurring [Member] | Total Fair Value [Member] | States, municipalities and political subdivisions
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading fixed maturity investments		41.7	354.1
Fair Value, Measurements, Recurring [Member] | Total Fair Value [Member] | Corporate Debt: Financial institutions
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading fixed maturity investments		1,046.4	1,032.7
Fair Value, Measurements, Recurring [Member] | Total Fair Value [Member] | Corporate debt: Industrials
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading fixed maturity investments		1,379.2	1,321.3
Fair Value, Measurements, Recurring [Member] | Total Fair Value [Member] | Corporate debt: Utilities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading fixed maturity investments		153.2	140.0
Fair Value, Measurements, Recurring [Member] | Total Fair Value [Member] | Agency mortgage-backed
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading fixed maturity investments		595.6	614.5
Fair Value, Measurements, Recurring [Member] | Total Fair Value [Member] | Non-agency residential mortgage-backed
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading fixed maturity investments		23.4	23.9
Fair Value, Measurements, Recurring [Member] | Total Fair Value [Member] | Commercial mortgage-backed
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading fixed maturity investments		467.1	598.0
Fair Value, Measurements, Recurring [Member] | Total Fair Value [Member] | Asset-backed
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading fixed maturity investments		$ 812.8	$ 757.3

[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.