Reinsurance Prepaid Reinsurance Premiums and Reinsurance Recoverables (Detail) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Reinsurance Premiums for Insurance Companies, by Product Segment [Line Items]
Ceded Premiums Written	$ 522.5	$ 260.2	$ 223.1
Ceded Premiums Earned	555.0	249.9	214.5
Prepaid reinsurance premiums	$ 199.3	$ 85.3
Percentage of Prepaid Reinsurance Premiums	100.00%	100.00%
Reinsurance Recoverables	$ 1,488.8	$ 1,231.9
Percentage of Reinsurance Recoverables	100.00%	100.00%
State Reinsurance Plan
Reinsurance Premiums for Insurance Companies, by Product Segment [Line Items]
Prepaid reinsurance premiums	$ 139.1	$ 81.2
Percentage of Prepaid Reinsurance Premiums	70.00%	95.00%
Reinsurance Recoverables	$ 1,421.5	$ 1,182.0
Percentage of Reinsurance Recoverables	96.00%	96.00%
State Reinsurance Plan \| Michigan Catastrophic Claims Association
Reinsurance Premiums for Insurance Companies, by Product Segment [Line Items]
Prepaid reinsurance premiums	$ 31.4	$ 32.8
Percentage of Prepaid Reinsurance Premiums	16.00%	38.00%
Reinsurance Recoverables	$ 1,217.6	$ 1,018.8
Percentage of Reinsurance Recoverables	82.00%	83.00%
State Reinsurance Plan \| Commercial Auto Insurance Procedures Plans
Reinsurance Premiums for Insurance Companies, by Product Segment [Line Items]
Prepaid reinsurance premiums	$ 37.1	$ 26.5
Percentage of Prepaid Reinsurance Premiums	19.00%	31.00%
Reinsurance Recoverables	$ 134.0	$ 110.1
Percentage of Reinsurance Recoverables	9.00%	9.00%
State Reinsurance Plan \| North Carolina Reinsurance Facility
Reinsurance Premiums for Insurance Companies, by Product Segment [Line Items]
Prepaid reinsurance premiums	$ 25.6	$ 21.9
Percentage of Prepaid Reinsurance Premiums	13.00%	26.00%
Reinsurance Recoverables	$ 56.7	$ 51.1
Percentage of Reinsurance Recoverables	4.00%	4.00%
State Reinsurance Plan \| other reinsurance program [Member]
Reinsurance Premiums for Insurance Companies, by Product Segment [Line Items]
Prepaid reinsurance premiums	$ 0.0	$ 0.0
Percentage of Prepaid Reinsurance Premiums	0.00%	0.00%
Reinsurance Recoverables	$ 2.8	$ 2.0
Percentage of Reinsurance Recoverables	0.00%	0.00%
Federal Reinsurance Plans [Member] \| National Flood Insurance Program [Member]
Reinsurance Premiums for Insurance Companies, by Product Segment [Line Items]
Prepaid reinsurance premiums	$ 45.0	$ 0.0
Percentage of Prepaid Reinsurance Premiums	22.00%	0.00%
Reinsurance Recoverables	$ 10.4	$ 0.0
Percentage of Reinsurance Recoverables	1.00%	0.00%
other reinsurance program [Member]
Reinsurance Premiums for Insurance Companies, by Product Segment [Line Items]
Prepaid reinsurance premiums	$ 7.6	$ 4.1
Percentage of Prepaid Reinsurance Premiums	4.00%	5.00%
Reinsurance Recoverables	$ 31.8	$ 49.9
Percentage of Reinsurance Recoverables	2.00%	4.00%
Non State Reinsurance Plans
Reinsurance Premiums for Insurance Companies, by Product Segment [Line Items]
Prepaid reinsurance premiums	$ 60.2	$ 4.1
Percentage of Prepaid Reinsurance Premiums	30.00%	5.00%
Reinsurance Recoverables	$ 67.3	$ 49.9
Percentage of Reinsurance Recoverables	4.00%	4.00%
Property reinsurance program [Member]
Reinsurance Premiums for Insurance Companies, by Product Segment [Line Items]
Prepaid reinsurance premiums	$ 52.6	$ 0.0
Percentage of Prepaid Reinsurance Premiums	26.00%	0.00%
Reinsurance Recoverables	$ 35.5	$ 0.0
Percentage of Reinsurance Recoverables	2.00%	0.00%
Regulated reinsurance plan [Member]
Reinsurance Premiums for Insurance Companies, by Product Segment [Line Items]
Ceded Premiums Written	$ 358.0	$ 251.9	216.2
Ceded Premiums Earned	362.6	241.4	205.4
non regulated reinsurance plan [Member]
Reinsurance Premiums for Insurance Companies, by Product Segment [Line Items]
Ceded Premiums Written	164.5	8.3	6.9
Ceded Premiums Earned	$ 192.4	$ 8.5	$ 9.1

Percentage of Total Prepaid Reinsurance Premiums

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- Definition

Percentage of Total Reinsurance Recoverables

+ References

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- 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)
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-Publisher FASB
-Name Accounting Standards Codification
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-Section S99
-Paragraph 2
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-URI http://asc.fasb.org/extlink&oid=34713774&loc=d3e574992-122915

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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 (b),(c)(2)
-URI http://asc.fasb.org/extlink&oid=6486672&loc=d3e27261-158547

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher SEC
-Name Regulation S-X (SX)
-Number 210
-Section 17
-Article 12

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The unexpired portion of premiums ceded on policies in force as of the balance sheet date.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Glossary Prepaid Reinsurance Premiums
-URI http://asc.fasb.org/extlink&oid=6521719

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 340
-Section 25
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=6485982&loc=d3e22818-158507

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Amount after valuation allowance recoverable including premium paid under reinsurance contracts. Examples include, but are not limited to, settled and unsettled claims, incurred but not reported losses, loss adjustment expense, premium paid, policy benefits and policy reserves.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 20
-Section 50
-Paragraph 5
-URI http://asc.fasb.org/extlink&oid=6480758&loc=d3e6824-158387

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