Investments (Schedule Of Other Investments) (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2016	Dec. 31, 2015
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Other invested assets		$ 960.7	$ 966.7
Unfunded commitments		605.5	613.8
Investments with redemption restrictions
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Other invested assets		559.8	502.0
Investments without redemption restrictions
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Other invested assets		$ 400.9	464.7
Hedge funds | Investments without redemption restrictions | Minimum
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Redemption notice period		30 days
Total Fair Value
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Other invested assets		$ 897.8	840.2
Unfunded commitments		605.5	613.8
Total Fair Value | Investments with redemption restrictions
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Other invested assets		559.8	502.0
Total Fair Value | Investments without redemption restrictions
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Other invested assets		338.0	338.2
Total Fair Value | Private equity funds
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Other invested assets		559.8	447.4
Unfunded commitments		605.5	613.8
Total Fair Value | Private equity funds | Investments with redemption restrictions
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Other invested assets		559.8	447.4
Total Fair Value | Private equity funds | Investments without redemption restrictions
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Other invested assets		0.0	0.0
Total Fair Value | Private equity (primary and secondary)
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Other invested assets		281.0	236.4
Unfunded commitments		219.3	231.0
Total Fair Value | Private equity (primary and secondary) | Investments with redemption restrictions
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Other invested assets		$ 281.0	$ 236.4
Total Fair Value | Private equity (primary and secondary) | Investments with redemption restrictions | Minimum
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Estimated remaining restriction period		1 year	1 year
Total Fair Value | Private equity (primary and secondary) | Investments with redemption restrictions | Maximum
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Estimated remaining restriction period		15 years	7 years
Total Fair Value | Private equity (primary and secondary) | Investments without redemption restrictions
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Other invested assets		$ 0.0	$ 0.0
Total Fair Value | Levered credit
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Other invested assets		204.9	205.9
Unfunded commitments		190.8	179.0
Total Fair Value | Levered credit | Investments with redemption restrictions
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Other invested assets		$ 204.9	$ 205.9
Total Fair Value | Levered credit | Investments with redemption restrictions | Minimum
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Estimated remaining restriction period		3 years	4 years
Total Fair Value | Levered credit | Investments with redemption restrictions | Maximum
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Estimated remaining restriction period		9 years	8 years
Total Fair Value | Levered credit | Investments without redemption restrictions
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Other invested assets		$ 0.0	$ 0.0
Total Fair Value | Distressed
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Other invested assets		5.0	5.1
Unfunded commitments		3.8	3.8
Total Fair Value | Distressed | Investments with redemption restrictions
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Other invested assets		$ 5.0	$ 5.1
Estimated remaining restriction period		1 year	2 years
Total Fair Value | Distressed | Investments without redemption restrictions
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Other invested assets		$ 0.0	$ 0.0
Total Fair Value | Real estate
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Other invested assets		68.9	0.0
Unfunded commitments		191.6	200.0
Total Fair Value | Real estate | Investments with redemption restrictions
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Other invested assets		$ 68.9	$ 0.0
Total Fair Value | Real estate | Investments with redemption restrictions | Minimum
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Estimated remaining restriction period		6 years	7 years
Total Fair Value | Real estate | Investments with redemption restrictions | Maximum
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Estimated remaining restriction period		8 years	9 years
Total Fair Value | Real estate | Investments without redemption restrictions
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Other invested assets		$ 0.0	$ 0.0
Total Fair Value | Hedge funds
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Other invested assets		338.0	379.0
Unfunded commitments		0.0	0.0
Total Fair Value | Hedge funds | Investments with redemption restrictions
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Other invested assets		0.0	54.6
Total Fair Value | Hedge funds | Investments without redemption restrictions
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Other invested assets		338.0	324.4
Total Fair Value | Distressed funds
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Other invested assets		175.3	215.6
Unfunded commitments		0.0	0.0
Total Fair Value | Distressed funds | Investments with redemption restrictions
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 | Distressed funds | Investments without redemption restrictions
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Other invested assets		$ 175.3	$ 161.0
Redemption frequency	[1]	Quarterly	Monthly
Redemption notice period	[1]	60 days	90 days
Total Fair Value | Relative value credit
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Other invested assets		$ 84.8	$ 105.4
Unfunded commitments		0.0	0.0
Total Fair Value | Relative value credit | Investments with redemption restrictions
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Other invested assets		0.0	0.0
Total Fair Value | Relative value credit | Investments without redemption restrictions
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Other invested assets		$ 84.8	$ 105.4
Redemption frequency	[1]	Quarterly	Quarterly
Redemption notice period	[1]	60 days	60 days
Total Fair Value | Equity long/short
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Other invested assets		$ 67.9	$ 58.0
Unfunded commitments		0.0	0.0
Total Fair Value | Equity long/short | Investments with redemption restrictions
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Other invested assets		0.0	0.0
Total Fair Value | Equity long/short | Investments without redemption restrictions
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Other invested assets		$ 67.9	$ 58.0
Redemption frequency	[1]	Quarterly	Quarterly
Redemption notice period	[1]	45 days	45 days
Total Fair Value | Fund of funds
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Other invested assets		$ 10.0
Unfunded commitments		0.0
Total Fair Value | Fund of funds | Investments with redemption restrictions
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Other invested assets		0.0
Total Fair Value | Fund of funds | Investments without redemption restrictions
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Other invested assets		$ 10.0
Redemption frequency	[1]	Quarterly
Redemption notice period	[1]	60 days
Total Fair Value | High yield loan fund
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 | High yield loan fund | Investments with redemption restrictions
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Other invested assets			0.0
Total Fair Value | High yield loan fund | Investments without redemption restrictions
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 | Other private securities
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Other invested assets		$ 62.9	$ 126.5
Unfunded commitments		0.0	0.0
Carrying Value | Other private securities | Investments with redemption restrictions
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Other invested assets		0.0	0.0
Carrying Value | Other private securities | Investments without redemption restrictions
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Other invested assets		$ 62.9	$ 126.5

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