Investments (Schedule Of Other Investments) (Details) - USD ($) $ in Millions		3 Months Ended	12 Months Ended
		Mar. 31, 2017	Dec. 31, 2016
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Other invested assets		$ 966.6	$ 960.7
Unfunded Commitments		593.4	605.5
Investments with redemption restrictions [Member]
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Other invested assets		566.4	559.8
Investments without redemption restrictions [Member]
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Other invested assets		$ 400.2	400.9
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		$ 902.7	897.8
Unfunded Commitments		593.4	605.5
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		566.4	559.8
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		336.3	338.0
Total Fair Value [Member] | Private Equity Funds [Member]
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Other invested assets		566.4	559.8
Unfunded Commitments		593.4	605.5
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		566.4	559.8
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		281.5	281.0
Unfunded Commitments		208.8	219.3
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		$ 281.5	$ 281.0
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		15 years	15 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		211.2	204.9
Unfunded Commitments		189.2	190.8
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		$ 211.2	$ 204.9
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		3 years	3 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		9 years	9 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] | Real Estate [Member]
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Other invested assets		68.9	68.9
Unfunded Commitments		191.6	191.6
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		$ 68.9	$ 68.9
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		6 years	6 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		8 years	8 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		4.8	5.0
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		$ 4.8	$ 5.0
Estimated remaining restriction period		1 year	1 year
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		336.3	338.0
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		0.0	0.0
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		336.3	338.0
Total Fair Value [Member] | Distressed Funds [Member]
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Other invested assets		173.3	175.3
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		0.0	0.0
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		$ 173.3	$ 175.3
Redemption frequency	[1]	Quarterly	Quarterly
Redemption notice period	[1]	60 days	60 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		$ 87.4	$ 84.8
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		$ 87.4	$ 84.8
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		$ 65.1	$ 67.9
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		$ 65.1	$ 67.9
Redemption frequency	[1]	Quarterly	Quarterly
Redemption notice period	[1]	45 days	45 days
Total Fair Value [Member] | Hedge Fund, Fund of Funds [Member]
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Other invested assets		$ 10.5	$ 10.0
Unfunded Commitments		0.0	0.0
Total Fair Value [Member] | Hedge Fund, 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	0.0
Total Fair Value [Member] | Hedge Fund, 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		$ 10.5	$ 10.0
Redemption frequency	[1]	Quarterly	Quarterly
Redemption notice period	[1]	60 days	60 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		$ 63.9	$ 62.9
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		$ 63.9	$ 62.9

[1] The redemption frequency and notice periods only apply to the investments without redemption restrictions.