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SCHEDULE I - Summary Of Investments - Other Than Investments in Related Parties Summary Of Investments - Other than Investments In Related Parties (Detail) $ in Millions	Dec. 31, 2017 USD ($)
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	$ 25,276.9
Fair Value	27,274.7
Amount At Which Shown In The Balance Sheet	27,274.7
Fixed maturities
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	20,209.9
Fair Value	20,201.7
Amount At Which Shown In The Balance Sheet	20,201.7
Fixed maturities | Bonds | US Government and Government Agencies and Authorities
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	6,688.8
Fair Value	6,645.9
Amount At Which Shown In The Balance Sheet	6,645.9
Fixed maturities | Bonds | State and local government obligations
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	2,285.6
Fair Value	2,297.1
Amount At Which Shown In The Balance Sheet	2,297.1
Fixed maturities | Bonds | Public Utility, Bonds
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	192.5
Fair Value	193.6
Amount At Which Shown In The Balance Sheet	193.6
Fixed maturities | Bonds | Corporate And Other Debt Securities
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	4,804.7
Fair Value	4,804.1
Amount At Which Shown In The Balance Sheet	4,804.1
Fixed maturities | Bonds | Asset-backed Securities
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	6,043.4
Fair Value	6,050.0
Amount At Which Shown In The Balance Sheet	6,050.0
Fixed maturities | Redeemable preferred stocks
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	194.9
Fair Value	211.0
Amount At Which Shown In The Balance Sheet	211.0
Equity securities
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	2,197.6
Fair Value	4,203.6
Amount At Which Shown In The Balance Sheet	4,203.6
Equity securities | Common Stocks, by Industry | Utilities
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	106.8
Fair Value	177.5
Amount At Which Shown In The Balance Sheet	177.5
Equity securities | Common Stocks, by Industry | Financials
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	285.8
Fair Value	685.0
Amount At Which Shown In The Balance Sheet	685.0
Equity securities | Common Stocks, by Industry | Industrials
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	1,106.4
Fair Value	2,537.3
Amount At Which Shown In The Balance Sheet	2,537.3
Equity securities | Nonredeemable preferred stocks
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	698.6
Fair Value	803.8
Amount At Which Shown In The Balance Sheet	803.8
Short-term investments
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	2,869.4
Fair Value	2,869.4
Amount At Which Shown In The Balance Sheet	$ 2,869.4

X

- Definition

This element represents the carrying (reported) amount of the entity's investments as of the balance sheet date. This amount may represent the individual investment's or group of similar investments' original cost, fair value, or such other amount at which the investment is shown in the entity's balance sheet.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 946
-SubTopic 320
-Section S99
-Paragraph 7
-Subparagraph (SX 210.12-15)
-URI http://asc.fasb.org/extlink&oid=50485924&loc=d3e611379-123010

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This item represents the original cost of the entity's investments including investments in equity securities and, as to fixed maturities, original cost reduced by repayments and adjusted for amortization of premiums or accrual of discounts.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 946
-SubTopic 320
-Section S99
-Paragraph 7
-Subparagraph (SX 210.12-15)
-URI http://asc.fasb.org/extlink&oid=50485924&loc=d3e611379-123010

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+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 946
-SubTopic 320
-Section S99
-Paragraph 7
-Subparagraph (SX 210.12-15)
-URI http://asc.fasb.org/extlink&oid=50485924&loc=d3e611379-123010

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+ References

No definition available.

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