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Concentration of Credit Risk - Schedule Of Gross Written Premium From Major Brokers (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Investment Holdings [Line Items]
Gross written premiums, percentage	100.00%	100.00%	100.00%
Gross written premiums	$ 3,360.9	$ 3,147.0	$ 2,997.3
Aon Corporation
Investment Holdings [Line Items]
Gross written premiums, percentage	16.40%	18.40%	18.70%
Marsh & McLennan Companies, Inc.
Investment Holdings [Line Items]
Gross written premiums, percentage	16.00%	14.70%	15.40%
Willis Group Holdings, Ltd.
Investment Holdings [Line Items]
Gross written premiums, percentage	13.10%	13.70%	14.50%
Others
Investment Holdings [Line Items]
Gross written premiums, percentage	54.50%	53.20%	51.40%

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Premiums written gross percentage.

+ References

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- Definition

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- Definition

Amount of direct and assumed premiums written.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 605
-Section 50
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=6486672&loc=d3e27261-158547

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