Statutory Financial Information (Detail) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Traditional U.S. Life Insurance Subsidiaries
Statutory Accounting Practices
Statutory Net Income (Loss)	$ 807.4	$ 855.1	$ 653.7
Statutory Combined Net Gain (Loss) from Operations	812.4	884.6	689.2
Statutory Combined Capital and Surplus	3,448.3	3,626.9
Captive Reinsurers
Statutory Accounting Practices
Statutory Net Income (Loss)	(137.1)	48.4	(56.3)
Statutory Combined Net Gain (Loss) from Operations	(143.1)	50.6	$ (54.3)
Statutory Combined Capital and Surplus	$ 1,606.9	$ 1,726.7

Statutory Accounting Practices Statutory Net Gain (Loss) from Operations

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- Definition

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.

+ References

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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://www.xbrl.org/2003/role/presentationRef
-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://www.xbrl.org/2003/role/presentationRef
-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.

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