Commitments And Contingencies (Credit Concentration Disclosure) (Details) $ in Millions	12 Months Ended
	Dec. 31, 2016 USD ($) Custodian	Dec. 31, 2015 Custodian	Dec. 31, 2014
Concentration Risk [Line Items]
Number of custodians | Custodian	1	1
Insurance balances receivable over 90 days	$ 20.0
Allowance for doubtful accounts	$ 3.0
Total receivable balance | Credit concentration risk
Concentration Risk [Line Items]
Concentration risk, percentage	2.50%
Gross written premium | Credit concentration risk | Marsh & McLennan Companies, Inc.
Concentration Risk [Line Items]
Concentration risk, percentage	21.00%	24.00%	25.00%
Gross written premium | Credit concentration risk | Aon Corporation
Concentration Risk [Line Items]
Concentration risk, percentage	14.00%	16.00%	17.00%
Gross written premium | Credit concentration risk | Willis Group Holdings
Concentration Risk [Line Items]
Concentration risk, percentage	9.00%	11.00%	10.00%