Investments (Schedule Of Other Investments) (Details) - USD ($) $ in Millions		9 Months Ended	12 Months Ended
		Sep. 30, 2016	Dec. 31, 2015
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Other invested assets		$ 941.7	$ 966.7
Investment commitment		610.0	613.8
Investments with redemption restrictions [Member]
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Other invested assets		495.6	502.0
Investments without redemption restrictions [Member]
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Other invested assets		$ 446.1	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		$ 827.1	840.2
Investment commitment		610.0	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		495.6	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		331.5	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		495.6	447.4
Investment commitment		610.0	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		495.6	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		248.1	236.4
Investment commitment		221.8	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		$ 248.1	$ 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] | Levered Credit [Member]
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Other invested assets		221.0	205.9
Investment commitment		195.1	179.0
Total Fair Value [Member] | Levered Credit [Member] | Investments with redemption restrictions [Member]
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Other invested assets		$ 221.0	$ 205.9
Total Fair Value [Member] | Levered Credit [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] | Levered Credit [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		8 years	8 years
Total Fair Value [Member] | Levered Credit [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		21.4	0.0
Investment commitment		189.3	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		$ 21.4	$ 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] | Distressed [Member]
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Other invested assets		5.1	5.1
Investment commitment		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		$ 5.1	$ 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] | Hedge Funds [Member]
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Other invested assets		331.5	379.0
Investment commitment		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		0.0	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		331.5	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		173.9	215.6
Investment commitment		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		0.0	$ 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		$ 173.9	$ 161.0
Redemption frequency	[1]	Monthly	Monthly
Redemption notice period	[1]	90 days	90 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		$ 82.6	$ 105.4
Investment commitment		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		$ 82.6	$ 105.4
Redemption frequency	[1]	Quarterly	Quarterly
Redemption notice period	[1]	60 days	60 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		$ 64.8	$ 58.0
Investment commitment		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		$ 64.8	$ 58.0
Redemption frequency	[1]	Quarterly	Quarterly
Redemption notice period	[1]	45 days	45 days
Total Fair Value [Member] | Fund of Funds [Member]
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Other invested assets		$ 10.2
Investment commitment		0.0
Total Fair Value [Member] | Fund of Funds [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] | Fund of Funds [Member] | Investments without redemption restrictions [Member]
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Other invested assets		$ 10.2
Redemption frequency	[1]	Annual
Redemption notice period	[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
Investment commitment			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 Private Securities [Member]
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Other invested assets		$ 114.6	$ 126.5
Investment commitment		0.0	0.0
Carrying Value [Member] | Other Private Securities [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
Carrying Value [Member] | Other Private Securities [Member] | Investments without redemption restrictions [Member]
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Other invested assets		$ 114.6	$ 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.