Investments (Schedule Of Other Investments) (Details) - USD ($) $ in Millions		3 Months Ended		12 Months Ended
		Mar. 31, 2016		Dec. 31, 2015
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Other invested assets		$ 940.3		$ 966.7
Unfunded Commitments		694.3		613.8
Investments with redemption restrictions [Member]
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Other invested assets		511.4		502.0
Investments without redemption restrictions [Member]
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Other invested assets		$ 428.9		464.7
Hedge Funds [Member] | Investments without redemption restrictions [Member] | Minimum [Member]
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Redemption notice period		30 days
Total Fair Value [Member]
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Other invested assets		$ 817.0		840.2
Unfunded Commitments		694.3		613.8
Total Fair Value [Member] | Investments with redemption restrictions [Member]
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Other invested assets		511.4		502.0
Total Fair Value [Member] | Investments without redemption restrictions [Member]
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Other invested assets		305.6		338.2
Total Fair Value [Member] | Private Equity Funds [Member]
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Other invested assets		454.9		447.4
Unfunded Commitments		694.3		613.8
Total Fair Value [Member] | Private Equity Funds [Member] | Investments with redemption restrictions [Member]
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Other invested assets		454.9		447.4
Total Fair Value [Member] | Private Equity Funds [Member] | Investments without redemption restrictions [Member]
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Other invested assets		0.0		0.0
Total Fair Value [Member] | Private Equity [Member]
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Other invested assets		240.1		236.4
Unfunded Commitments		223.4		231.0
Total Fair Value [Member] | Private Equity [Member] | Investments with redemption restrictions [Member]
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Other invested assets		$ 240.1		$ 236.4
Total Fair Value [Member] | Private Equity [Member] | Investments with redemption restrictions [Member] | Minimum [Member]
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Estimated remaining restriction period		1 year		1 year
Total Fair Value [Member] | Private Equity [Member] | Investments with redemption restrictions [Member] | Maximum [Member]
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Estimated remaining restriction period		7 years		7 years
Total Fair Value [Member] | Private Equity [Member] | Investments without redemption restrictions [Member]
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Other invested assets		$ 0.0		$ 0.0
Total Fair Value [Member] | Mezzanine Debt [Member]
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Other invested assets		209.6		205.9
Unfunded Commitments		217.1		179.0
Total Fair Value [Member] | Mezzanine Debt [Member] | Investments with redemption restrictions [Member]
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Other invested assets		$ 209.6		$ 205.9
Total Fair Value [Member] | Mezzanine Debt [Member] | Investments with redemption restrictions [Member] | Minimum [Member]
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Estimated remaining restriction period		4 years		4 years
Total Fair Value [Member] | Mezzanine Debt [Member] | Investments with redemption restrictions [Member] | Maximum [Member]
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Estimated remaining restriction period		10 years		8 years
Total Fair Value [Member] | Mezzanine Debt [Member] | Investments without redemption restrictions [Member]
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Other invested assets		$ 0.0		$ 0.0
Total Fair Value [Member] | Distressed [Member]
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Other invested assets		5.2		5.1
Unfunded Commitments		3.8		3.8
Total Fair Value [Member] | Distressed [Member] | Investments with redemption restrictions [Member]
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Other invested assets		$ 5.2		$ 5.1
Estimated remaining restriction period		2 years		2 years
Total Fair Value [Member] | Distressed [Member] | Investments without redemption restrictions [Member]
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Other invested assets		$ 0.0		$ 0.0
Total Fair Value [Member] | Real Estate [Member]
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Other invested assets		0.0		0.0
Unfunded Commitments		250.0		200.0
Total Fair Value [Member] | Real Estate [Member] | Investments with redemption restrictions [Member]
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Other invested assets		$ 0.0		$ 0.0
Total Fair Value [Member] | Real Estate [Member] | Investments with redemption restrictions [Member] | Minimum [Member]
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Estimated remaining restriction period		7 years		7 years
Total Fair Value [Member] | Real Estate [Member] | Investments with redemption restrictions [Member] | Maximum [Member]
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Estimated remaining restriction period		9 years		9 years
Total Fair Value [Member] | Real Estate [Member] | Investments without redemption restrictions [Member]
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Other invested assets		$ 0.0		$ 0.0
Total Fair Value [Member] | Hedge Funds [Member]
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Other invested assets		362.1		379.0
Unfunded Commitments		0.0		0.0
Total Fair Value [Member] | Hedge Funds [Member] | Investments with redemption restrictions [Member]
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Other invested assets		56.5		54.6
Total Fair Value [Member] | Hedge Funds [Member] | Investments without redemption restrictions [Member]
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Other invested assets		305.6		324.4
Total Fair Value [Member] | Distressed Funds [Member]
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Other invested assets		216.2		215.6
Unfunded Commitments		0.0		0.0
Total Fair Value [Member] | Distressed Funds [Member] | Investments with redemption restrictions [Member]
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Other invested assets		$ 56.5		54.6
Estimated remaining restriction period		2 years
Total Fair Value [Member] | Distressed Funds [Member] | Investments without redemption restrictions [Member]
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Other invested assets		$ 159.7		$ 161.0
Redemption frequency	[1]	Monthly		Monthly
Redemption notice period	[1]	90 days		90 days
Total Fair Value [Member] | Equity Long/Short [Member]
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Other invested assets		$ 58.8		$ 58.0
Unfunded Commitments		0.0		0.0
Total Fair Value [Member] | Equity Long/Short [Member] | Investments with redemption restrictions [Member]
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Other invested assets		0.0		0.0
Total Fair Value [Member] | Equity Long/Short [Member] | Investments without redemption restrictions [Member]
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Other invested assets		$ 58.8		$ 58.0
Redemption frequency	[1]	Quarterly		Quarterly
Redemption notice period	[1]	45 days		45 days
Total Fair Value [Member] | Relative Value Credit [Member]
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Other invested assets		$ 87.1		$ 105.4
Unfunded Commitments		0.0		0.0
Total Fair Value [Member] | Relative Value Credit [Member] | Investments with redemption restrictions [Member]
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Other invested assets		0.0		0.0
Total Fair Value [Member] | Relative Value Credit [Member] | Investments without redemption restrictions [Member]
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Other invested assets		$ 87.1		$ 105.4
Redemption frequency	[1]	Quarterly		Quarterly
Redemption notice period		60 days	[1]	60 days
Total Fair Value [Member] | High Yield Loan Fund [Member]
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Other invested assets				$ 13.8
Unfunded Commitments				0.0
Total Fair Value [Member] | High Yield Loan Fund [Member] | Investments with redemption restrictions [Member]
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Other invested assets				0.0
Total Fair Value [Member] | High Yield Loan Fund [Member] | Investments without redemption restrictions [Member]
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Other invested assets				$ 13.8
Redemption frequency	[1]			Monthly
Redemption notice period	[1]			30 days
Carrying Value [Member] | Other invested assets: other private securities
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Other invested assets		$ 123.3		$ 126.5
Unfunded Commitments		0.0		0.0
Carrying Value [Member] | Other invested assets: other private securities | Investments with redemption restrictions [Member]
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Other invested assets		0.0		0.0
Carrying Value [Member] | Other invested assets: other private securities | Investments without redemption restrictions [Member]
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Other invested assets		$ 123.3		$ 126.5

[1] The redemption frequency and notice periods only apply to the investments without redemption restrictions. Some or all of these investments may be subject to a gate as described below.