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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 ($)
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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

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- 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 + Details
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- Definition 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 + Details
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- Definition This item represents the reported amount of the entity's investments measured at fair value. Fair value is the price that would be received to sell an investment in an orderly transaction between market participants at the balance sheet [measurement] date. + 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 + Details
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