Summary of Statutory Net Income (Loss) and Capital and Surplus (Detail) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
NIC and NSIC
Statutory Accounting Practices [Line Items]
Net Income	$ 75.0	$ 79.3	$ 84.2
Capital and Surplus	1,005.4	1,056.6	1,027.3
Syndicate
Statutory Accounting Practices [Line Items]
Net Income	6.8	(23.4)	46.3
Capital and Surplus	165.3	168.7	180.7
NIIC
Statutory Accounting Practices [Line Items]
Net Income	(14.7)	(8.4)	0.4
Capital and Surplus	$ 43.3	$ 62.9	$ 70.7

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

No definition available.

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- Definition

The amount of statutory capital and surplus (stockholders' equity) as of the balance sheet date using prescribed or permitted statutory accounting practices (rather than GAAP, if different) of the state or country.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03(a)(23)(c))
-URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 505
-Section 50
-Paragraph 1
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=6486098&loc=d3e24708-158529

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- Definition

Amount of net income for the period determined using accounting principles prescribed or permitted by insurance regulators.

+ References

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