Reserve For Losses And Loss Expenses (Summary of Claims Development and Reinsurance Recoverable) (Details) - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Liability for losses and loss expenses, net of reinsurance	$ 4,900.4
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments	1,574.0
Total gross reserves for losses and loss expenses per the loss development triangles:	6,474.4
Unallocated loss adjustment expenses	118.6
Other reserves not included in the loss development triangles	43.6
Fair value adjustment	6.0
Currency translation adjustment	(3.0)
Other	(0.4)
Other balances not included in the loss development triangles	164.8
Reserve for losses and loss expenses	6,639.2	$ 6,456.2	$ 5,881.2	$ 5,766.5
General casualty | North American Insurance
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Liability for losses and loss expenses, net of reinsurance	1,079.9
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments	605.9
Other reserves not included in the loss development triangles	157.0
Professional liability | North American Insurance
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Liability for losses and loss expenses, net of reinsurance	891.6
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments	352.7
Other reserves not included in the loss development triangles	89.7
General property | North American Insurance
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Liability for losses and loss expenses, net of reinsurance	79.0
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments	74.5
Other reserves not included in the loss development triangles	0.5
Programs | North American Insurance
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Liability for losses and loss expenses, net of reinsurance	265.8
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments	83.8
Other reserves not included in the loss development triangles	9.4
Healthcare | North American Insurance
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Liability for losses and loss expenses, net of reinsurance	301.2
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments	135.6
Other reserves not included in the loss development triangles	16.8
Specialty | North American Insurance
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Liability for losses and loss expenses, net of reinsurance	134.0
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments	55.6
Other reserves not included in the loss development triangles	0.0
Property | Reinsurance
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Liability for losses and loss expenses, net of reinsurance	245.7
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments	3.8
Other reserves not included in the loss development triangles	1.9
Casualty | Reinsurance
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Liability for losses and loss expenses, net of reinsurance	1,030.3
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments	0.0
Other reserves not included in the loss development triangles	225.9
Specialty and other | Reinsurance
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Liability for losses and loss expenses, net of reinsurance	175.7
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments	0.0
Other reserves not included in the loss development triangles	3.3
Excluding Acquired Reserves [Member] | General casualty | Global Markets Insurance
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Liability for losses and loss expenses, net of reinsurance	88.3
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments	62.9
Other reserves not included in the loss development triangles	24.1
Excluding Acquired Reserves [Member] | Professional liability | Global Markets Insurance
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Liability for losses and loss expenses, net of reinsurance	246.2
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments	111.9
Other reserves not included in the loss development triangles	46.5
Excluding Acquired Reserves [Member] | General property | Global Markets Insurance
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Liability for losses and loss expenses, net of reinsurance	47.8
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments	4.0
Other reserves not included in the loss development triangles	2.3
Excluding Acquired Reserves [Member] | Specialty | Global Markets Insurance
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Liability for losses and loss expenses, net of reinsurance	67.9
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments	30.7
Other reserves not included in the loss development triangles	0.0
Acquired Reserves [Member] | General casualty | Global Markets Insurance
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Liability for losses and loss expenses, net of reinsurance	148.6
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments	5.0
Other reserves not included in the loss development triangles	1.1
Acquired Reserves [Member] | Professional liability | Global Markets Insurance
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Liability for losses and loss expenses, net of reinsurance	6.8
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments	1.0
Other reserves not included in the loss development triangles	0.0
Acquired Reserves [Member] | General property | Global Markets Insurance
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Liability for losses and loss expenses, net of reinsurance	25.0
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments	15.3
Other reserves not included in the loss development triangles	(3.5)
Acquired Reserves [Member] | Specialty | Global Markets Insurance
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Liability for losses and loss expenses, net of reinsurance	66.7
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments	31.5
Other reserves not included in the loss development triangles	$ 0.4