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Reinsurance Effect of Reinsurance on Premiums Written and Earned (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Reinsurance Premiums for Insurance Companies, by Product Segment [Line Items]
Direct premiums written	$ 27,860.7	$ 23,941.9	$ 21,086.5
Ceded written	(728.6)	(588.4)	(522.5)
Net premiums written	27,132.1	23,353.5	20,564.0
Direct premiums earned	26,425.7	23,111.2	20,454.1
Ceded earned	(695.8)	(637.2)	(555.0)
Net premiums earned	25,729.9	22,474.0	19,899.1
non regulated reinsurance plan [Member]
Reinsurance Premiums for Insurance Companies, by Product Segment [Line Items]
Ceded written	(222.7)	(149.0)	(164.5)
Ceded earned	(216.2)	(212.1)	(192.4)
Regulated reinsurance plan [Member]
Reinsurance Premiums for Insurance Companies, by Product Segment [Line Items]
Ceded written	(505.9)	(439.4)	(358.0)
Ceded earned	$ (479.6)	$ (425.1)	$ (362.6)

X

- Definition

Amount of earned premiums ceded to other entities.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 225
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-04.1)
-URI http://asc.fasb.org/extlink&oid=6879464&loc=d3e573970-122913

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-Publisher FASB
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-SubTopic 605
-Section 45
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-Publisher FASB
-Name Accounting Standards Codification
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Amount of written premiums ceded to other entities.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 605
-Section 50
-Paragraph 1
-Subparagraph (c)(2)
-URI http://asc.fasb.org/extlink&oid=6486672&loc=d3e27261-158547

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-Publisher FASB
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+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 605
-Section 50
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+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
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-SubTopic 605
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+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
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+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
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No definition available.

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