Fair Value Composition of Investment Portfolio by Major Security Type (Detail) - USD ($) $ in Millions | Jun. 30, 2017 | Dec. 31, 2016 | Jun. 30, 2016 |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | | |
Available-for-sale Securities | $ 25,978.4 | $ 23,482.6 | $ 22,379.8 |
Long-Term Debt, Fair Value | 3,679.2 | 3,315.8 | 2,982.1 |
Fixed maturities | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | | |
Available-for-sale Securities | 18,388.1 | 16,243.8 | 13,589.5 |
Fixed maturities | Debt Securities [Member] | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | | |
Available-for-sale Securities | 11,691.4 | 9,948.1 | 7,849.6 |
Fixed maturities | Asset-backed Securities | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | | |
Available-for-sale Securities | 6,490 | 6,103.8 | 5,503.8 |
Fixed maturities | U.S. government obligations | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | | |
Available-for-sale Securities | 4,155 | 2,870.1 | 1,296 |
Fixed maturities | U.S. government obligations | Debt Securities [Member] | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | | |
Available-for-sale Securities | 4,155 | 2,870.1 | 1,296 |
Fixed maturities | State and local government obligations | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | | |
Available-for-sale Securities | 2,500.1 | 2,502.6 | 2,617.6 |
Fixed maturities | State and local government obligations | Debt Securities [Member] | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | | |
Available-for-sale Securities | 2,500.1 | 2,502.6 | 2,617.6 |
Fixed maturities | Foreign government obligations | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | | |
Available-for-sale Securities | 22.5 | 24.5 | 25 |
Fixed maturities | Foreign government obligations | Debt Securities [Member] | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | | |
Available-for-sale Securities | 22.5 | 24.5 | 25 |
Fixed maturities | Corporate debt securities | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | | |
Available-for-sale Securities | 5,013.8 | 4,550.9 | 3,911 |
Fixed maturities | Corporate debt securities | Debt Securities [Member] | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | | |
Available-for-sale Securities | 5,013.8 | 4,550.9 | 3,911 |
Fixed maturities | Residential mortgage-backed securities | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | | |
Available-for-sale Securities | 1,230.6 | 1,458.7 | 1,674.9 |
Fixed maturities | Residential mortgage-backed securities | Asset-backed Securities | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | | |
Available-for-sale Securities | 1,230.6 | 1,458.7 | 1,674.9 |
Fixed maturities | Agency residential pass-through obligations | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | | |
Available-for-sale Securities | 37 | 40.6 | 46.9 |
Fixed maturities | Agency residential pass-through obligations | Asset-backed Securities | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | | |
Available-for-sale Securities | 37 | 40.6 | 46.9 |
Fixed maturities | Commercial mortgage-backed securities | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | | |
Available-for-sale Securities | 2,375 | 2,253.4 | 2,210.4 |
Fixed maturities | Commercial mortgage-backed securities | Asset-backed Securities | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | | |
Available-for-sale Securities | 2,375 | 2,253.4 | 2,210.4 |
Fixed maturities | Other asset-backed securities | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | | |
Available-for-sale Securities | 2,847.4 | 2,351.1 | 1,571.6 |
Fixed maturities | Other asset-backed securities | Asset-backed Securities | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | | |
Available-for-sale Securities | 2,847.4 | 2,351.1 | 1,571.6 |
Fixed maturities | Redeemable preferred stocks | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | | |
Available-for-sale Securities | 206.7 | 191.9 | 236.1 |
Fixed maturities | Redeemable preferred stocks | Financials | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | | |
Available-for-sale Securities | 64.1 | 59.5 | 93.2 |
Fixed maturities | Redeemable preferred stocks | Utilities | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | | |
Available-for-sale Securities | 32.2 | 30.9 | 53.8 |
Fixed maturities | Redeemable preferred stocks | Industrials | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | | |
Available-for-sale Securities | 110.4 | 101.5 | 89.1 |
Equity securities | Nonredeemable preferred stocks | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | | |
Available-for-sale Securities | 783.1 | 853.5 | 858.5 |
Equity securities | Nonredeemable preferred stocks | Financials | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | | |
Available-for-sale Securities | 783.1 | 853.5 | 858.5 |
Equity securities | Common equities | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | | |
Available-for-sale Securities | 3,077.5 | 2,812.4 | 2,765.4 |
Equity securities | Common equities | Common equities | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | | |
Available-for-sale Securities | 3,077.2 | 2,812 | 2,765.1 |
Equity securities | Common equities | Other risk investments | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | | |
Available-for-sale Securities | 0.3 | 0.4 | 0.3 |
Total Fixed Maturities and Equity Securities | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | | |
Available-for-sale Securities | 22,248.7 | 19,909.7 | 17,213.4 |
Short-term investments | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | | |
Available-for-sale Securities | 3,729.7 | 3,572.9 | 5,166.4 |
Carrying (Reported) Amount, Fair Value Disclosure | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | | |
Available-for-sale Securities | 24,179.7 | 22,031.7 | 20,880.1 |
Long-term Debt | 3,383.4 | 3,148.2 | 2,664.1 |
Carrying (Reported) Amount, Fair Value Disclosure | Fixed maturities | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | | |
Available-for-sale Securities | 18,311.7 | 16,287.1 | 13,409.6 |
Carrying (Reported) Amount, Fair Value Disclosure | Fixed maturities | Debt Securities [Member] | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | | |
Available-for-sale Securities | 11,653.8 | 9,991 | 7,680.3 |
Carrying (Reported) Amount, Fair Value Disclosure | Fixed maturities | Asset-backed Securities | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | | |
Available-for-sale Securities | 6,468.5 | 6,107.3 | 5,464.2 |
Carrying (Reported) Amount, Fair Value Disclosure | Fixed maturities | U.S. government obligations | Debt Securities [Member] | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | | |
Available-for-sale Securities | 4,166.4 | 2,899.2 | 1,276 |
Carrying (Reported) Amount, Fair Value Disclosure | Fixed maturities | State and local government obligations | Debt Securities [Member] | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | | |
Available-for-sale Securities | 2,473.3 | 2,509.5 | 2,545.5 |
Carrying (Reported) Amount, Fair Value Disclosure | Fixed maturities | Foreign government obligations | Debt Securities [Member] | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | | |
Available-for-sale Securities | 22.5 | 24.5 | 24.9 |
Carrying (Reported) Amount, Fair Value Disclosure | Fixed maturities | Corporate debt securities | Debt Securities [Member] | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | | |
Available-for-sale Securities | 4,991.6 | 4,557.8 | 3,833.9 |
Carrying (Reported) Amount, Fair Value Disclosure | Fixed maturities | Residential mortgage-backed securities | Asset-backed Securities | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | | |
Available-for-sale Securities | 1,222.8 | 1,448.5 | 1,672.8 |
Carrying (Reported) Amount, Fair Value Disclosure | Fixed maturities | Agency residential pass-through obligations | Asset-backed Securities | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | | |
Available-for-sale Securities | 37.5 | 41.2 | 46.4 |
Carrying (Reported) Amount, Fair Value Disclosure | Fixed maturities | Commercial mortgage-backed securities | Asset-backed Securities | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | | |
Available-for-sale Securities | 2,364.9 | 2,266.9 | 2,177.3 |
Carrying (Reported) Amount, Fair Value Disclosure | Fixed maturities | Other asset-backed securities | Asset-backed Securities | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | | |
Available-for-sale Securities | 2,843.3 | 2,350.7 | 1,567.7 |
Carrying (Reported) Amount, Fair Value Disclosure | Fixed maturities | Redeemable preferred stocks | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | | |
Available-for-sale Securities | 189.4 | 188.8 | 265.1 |
Carrying (Reported) Amount, Fair Value Disclosure | Fixed maturities | Redeemable preferred stocks | Financials | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | | |
Available-for-sale Securities | 60.4 | 59.8 | 81.7 |
Carrying (Reported) Amount, Fair Value Disclosure | Fixed maturities | Redeemable preferred stocks | Utilities | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | | |
Available-for-sale Securities | 30.5 | 30.5 | 65.2 |
Carrying (Reported) Amount, Fair Value Disclosure | Fixed maturities | Redeemable preferred stocks | Industrials | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | | |
Available-for-sale Securities | 98.5 | 98.5 | 118.2 |
Carrying (Reported) Amount, Fair Value Disclosure | Equity securities | Nonredeemable preferred stocks | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | | |
Available-for-sale Securities | 666 | 734.2 | 745.5 |
Carrying (Reported) Amount, Fair Value Disclosure | Equity securities | Nonredeemable preferred stocks | Financials | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | | |
Available-for-sale Securities | 666 | 734.2 | 745.5 |
Carrying (Reported) Amount, Fair Value Disclosure | Equity securities | Common equities | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | | |
Available-for-sale Securities | 1,472.3 | 1,437.5 | 1,558.6 |
Carrying (Reported) Amount, Fair Value Disclosure | Equity securities | Common equities | Common equities | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | | |
Available-for-sale Securities | 1,472 | 1,437.1 | 1,558.3 |
Carrying (Reported) Amount, Fair Value Disclosure | Equity securities | Common equities | Other risk investments | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | | |
Available-for-sale Securities | 0.3 | 0.4 | 0.3 |
Carrying (Reported) Amount, Fair Value Disclosure | Total Fixed Maturities and Equity Securities | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | | |
Available-for-sale Securities | 20,450 | 18,458.8 | 15,713.7 |
Carrying (Reported) Amount, Fair Value Disclosure | Short-term investments | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | | |
Available-for-sale Securities | 3,729.7 | 3,572.9 | 5,166.4 |
Fair Value, Inputs, Level 1 | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | | |
Available-for-sale Securities | 9,788 | 8,854 | 9,283.8 |
Long-Term Debt, Fair Value | 0 | 0 | 0 |
Fair Value, Inputs, Level 1 | Fixed maturities | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | | |
Available-for-sale Securities | 4,177.5 | 2,894.6 | 1,321 |
Fair Value, Inputs, Level 1 | Fixed maturities | Debt Securities [Member] | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | | |
Available-for-sale Securities | 4,177.5 | 2,894.6 | 1,321 |
Fair Value, Inputs, Level 1 | Fixed maturities | Asset-backed Securities | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | | |
Available-for-sale Securities | 0 | 0 | 0 |
Fair Value, Inputs, Level 1 | Fixed maturities | U.S. government obligations | Debt Securities [Member] | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | | |
Available-for-sale Securities | 4,155 | 2,870.1 | 1,296 |
Fair Value, Inputs, Level 1 | Fixed maturities | State and local government obligations | Debt Securities [Member] | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | | |
Available-for-sale Securities | 0 | 0 | 0 |
Fair Value, Inputs, Level 1 | Fixed maturities | Foreign government obligations | Debt Securities [Member] | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | | |
Available-for-sale Securities | 22.5 | 24.5 | 25 |
Fair Value, Inputs, Level 1 | Fixed maturities | Corporate debt securities | Debt Securities [Member] | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | | |
Available-for-sale Securities | 0 | 0 | 0 |
Fair Value, Inputs, Level 1 | Fixed maturities | Residential mortgage-backed securities | Asset-backed Securities | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | | |
Available-for-sale Securities | 0 | 0 | 0 |
Fair Value, Inputs, Level 1 | Fixed maturities | Agency residential pass-through obligations | Asset-backed Securities | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | | |
Available-for-sale Securities | 0 | 0 | 0 |
Fair Value, Inputs, Level 1 | Fixed maturities | Commercial mortgage-backed securities | Asset-backed Securities | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | | |
Available-for-sale Securities | 0 | 0 | 0 |
Fair Value, Inputs, Level 1 | Fixed maturities | Other asset-backed securities | Asset-backed Securities | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | | |
Available-for-sale Securities | 0 | 0 | 0 |
Fair Value, Inputs, Level 1 | Fixed maturities | Redeemable preferred stocks | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | | |
Available-for-sale Securities | 0 | 0 | 0 |
Fair Value, Inputs, Level 1 | Fixed maturities | Redeemable preferred stocks | Financials | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | | |
Available-for-sale Securities | 0 | 0 | 0 |
Fair Value, Inputs, Level 1 | Fixed maturities | Redeemable preferred stocks | Utilities | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | | |
Available-for-sale Securities | 0 | 0 | 0 |
Fair Value, Inputs, Level 1 | Fixed maturities | Redeemable preferred stocks | Industrials | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | | |
Available-for-sale Securities | 0 | 0 | 0 |
Fair Value, Inputs, Level 1 | Equity securities | Nonredeemable preferred stocks | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | | |
Available-for-sale Securities | 85.3 | 138.1 | 156.5 |
Fair Value, Inputs, Level 1 | Equity securities | Nonredeemable preferred stocks | Financials | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | | |
Available-for-sale Securities | 85.3 | 138.1 | 156.5 |
Fair Value, Inputs, Level 1 | Equity securities | Common equities | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | | |
Available-for-sale Securities | 3,077.2 | 2,812 | 2,765.1 |
Fair Value, Inputs, Level 1 | Equity securities | Common equities | Common equities | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | | |
Available-for-sale Securities | 3,077.2 | 2,812 | 2,765.1 |
Fair Value, Inputs, Level 1 | Equity securities | Common equities | Other risk investments | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | | |
Available-for-sale Securities | 0 | 0 | 0 |
Fair Value, Inputs, Level 1 | Total Fixed Maturities and Equity Securities | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | | |
Available-for-sale Securities | 7,340 | 5,844.7 | 4,242.6 |
Fair Value, Inputs, Level 1 | Short-term investments | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | | |
Available-for-sale Securities | 2,448 | 3,009.3 | 5,041.2 |
Fair Value, Inputs, Level 2 | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | | |
Available-for-sale Securities | 16,190.1 | 14,627.9 | 13,086.5 |
Long-Term Debt, Fair Value | 3,564.4 | 3,188.5 | 2,842.3 |
Fair Value, Inputs, Level 2 | Fixed maturities | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | | |
Available-for-sale Securities | 14,210.6 | 13,348.9 | 12,259.3 |
Fair Value, Inputs, Level 2 | Fixed maturities | Debt Securities [Member] | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | | |
Available-for-sale Securities | 7,513.9 | 7,053.5 | 6,528.6 |
Fair Value, Inputs, Level 2 | Fixed maturities | Asset-backed Securities | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | | |
Available-for-sale Securities | 6,490 | 6,103.5 | 5,494.6 |
Fair Value, Inputs, Level 2 | Fixed maturities | U.S. government obligations | Debt Securities [Member] | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | | |
Available-for-sale Securities | 0 | 0 | 0 |
Fair Value, Inputs, Level 2 | Fixed maturities | State and local government obligations | Debt Securities [Member] | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | | |
Available-for-sale Securities | 2,500.1 | 2,502.6 | 2,617.6 |
Fair Value, Inputs, Level 2 | Fixed maturities | Foreign government obligations | Debt Securities [Member] | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | | |
Available-for-sale Securities | 0 | 0 | 0 |
Fair Value, Inputs, Level 2 | Fixed maturities | Corporate debt securities | Debt Securities [Member] | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | | |
Available-for-sale Securities | 5,013.8 | 4,550.9 | 3,911 |
Fair Value, Inputs, Level 2 | Fixed maturities | Residential mortgage-backed securities | Asset-backed Securities | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | | |
Available-for-sale Securities | 1,230.6 | 1,458.7 | 1,674.9 |
Fair Value, Inputs, Level 2 | Fixed maturities | Agency residential pass-through obligations | Asset-backed Securities | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | | |
Available-for-sale Securities | 37 | 40.6 | 46.9 |
Fair Value, Inputs, Level 2 | Fixed maturities | Commercial mortgage-backed securities | Asset-backed Securities | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | | |
Available-for-sale Securities | 2,375 | 2,253.1 | 2,201.2 |
Fair Value, Inputs, Level 2 | Fixed maturities | Other asset-backed securities | Asset-backed Securities | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | | |
Available-for-sale Securities | 2,847.4 | 2,351.1 | 1,571.6 |
Fair Value, Inputs, Level 2 | Fixed maturities | Redeemable preferred stocks | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | | |
Available-for-sale Securities | 206.7 | 191.9 | 236.1 |
Fair Value, Inputs, Level 2 | Fixed maturities | Redeemable preferred stocks | Financials | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | | |
Available-for-sale Securities | 64.1 | 59.5 | 93.2 |
Fair Value, Inputs, Level 2 | Fixed maturities | Redeemable preferred stocks | Utilities | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | | |
Available-for-sale Securities | 32.2 | 30.9 | 53.8 |
Fair Value, Inputs, Level 2 | Fixed maturities | Redeemable preferred stocks | Industrials | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | | |
Available-for-sale Securities | 110.4 | 101.5 | 89.1 |
Fair Value, Inputs, Level 2 | Equity securities | Nonredeemable preferred stocks | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | | |
Available-for-sale Securities | 697.8 | 715.4 | 702 |
Fair Value, Inputs, Level 2 | Equity securities | Nonredeemable preferred stocks | Financials | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | | |
Available-for-sale Securities | 697.8 | 715.4 | 702 |
Fair Value, Inputs, Level 2 | Equity securities | Common equities | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | | |
Available-for-sale Securities | 0 | 0 | 0 |
Fair Value, Inputs, Level 2 | Equity securities | Common equities | Common equities | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | | |
Available-for-sale Securities | 0 | 0 | 0 |
Fair Value, Inputs, Level 2 | Equity securities | Common equities | Other risk investments | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | | |
Available-for-sale Securities | 0 | 0 | 0 |
Fair Value, Inputs, Level 2 | Total Fixed Maturities and Equity Securities | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | | |
Available-for-sale Securities | 14,908.4 | 14,064.3 | 12,961.3 |
Fair Value, Inputs, Level 2 | Short-term investments | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | | |
Available-for-sale Securities | 1,281.7 | 563.6 | 125.2 |
Fair Value, Inputs, Level 3 | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | | |
Available-for-sale Securities | 0.3 | 0.7 | 9.5 |
Long-Term Debt, Fair Value | 114.8 | 127.3 | 139.8 |
Fair Value, Inputs, Level 3 | Fixed maturities | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | | |
Available-for-sale Securities | 0 | 0.3 | 9.2 |
Fair Value, Inputs, Level 3 | Fixed maturities | Debt Securities [Member] | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | | |
Available-for-sale Securities | 0 | 0 | 0 |
Fair Value, Inputs, Level 3 | Fixed maturities | Asset-backed Securities | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | | |
Available-for-sale Securities | 0 | 0.3 | 9.2 |
Fair Value, Inputs, Level 3 | Fixed maturities | U.S. government obligations | Debt Securities [Member] | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | | |
Available-for-sale Securities | 0 | 0 | 0 |
Fair Value, Inputs, Level 3 | Fixed maturities | State and local government obligations | Debt Securities [Member] | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | | |
Available-for-sale Securities | 0 | 0 | 0 |
Fair Value, Inputs, Level 3 | Fixed maturities | Foreign government obligations | Debt Securities [Member] | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | | |
Available-for-sale Securities | 0 | 0 | 0 |
Fair Value, Inputs, Level 3 | Fixed maturities | Corporate debt securities | Debt Securities [Member] | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | | |
Available-for-sale Securities | 0 | 0 | 0 |
Fair Value, Inputs, Level 3 | Fixed maturities | Residential mortgage-backed securities | Asset-backed Securities | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | | |
Available-for-sale Securities | 0 | 0 | 0 |
Fair Value, Inputs, Level 3 | Fixed maturities | Agency residential pass-through obligations | Asset-backed Securities | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | | |
Available-for-sale Securities | 0 | 0 | 0 |
Fair Value, Inputs, Level 3 | Fixed maturities | Commercial mortgage-backed securities | Asset-backed Securities | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | | |
Available-for-sale Securities | 0 | 0.3 | 9.2 |
Fair Value, Inputs, Level 3 | Fixed maturities | Other asset-backed securities | Asset-backed Securities | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | | |
Available-for-sale Securities | 0 | 0 | 0 |
Fair Value, Inputs, Level 3 | Fixed maturities | Redeemable preferred stocks | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | | |
Available-for-sale Securities | 0 | 0 | 0 |
Fair Value, Inputs, Level 3 | Fixed maturities | Redeemable preferred stocks | Financials | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | | |
Available-for-sale Securities | 0 | 0 | 0 |
Fair Value, Inputs, Level 3 | Fixed maturities | Redeemable preferred stocks | Utilities | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | | |
Available-for-sale Securities | 0 | 0 | 0 |
Fair Value, Inputs, Level 3 | Fixed maturities | Redeemable preferred stocks | Industrials | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | | |
Available-for-sale Securities | 0 | 0 | 0 |
Fair Value, Inputs, Level 3 | Equity securities | Nonredeemable preferred stocks | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | | |
Available-for-sale Securities | 0 | 0 | 0 |
Fair Value, Inputs, Level 3 | Equity securities | Nonredeemable preferred stocks | Financials | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | | |
Available-for-sale Securities | 0 | 0 | 0 |
Fair Value, Inputs, Level 3 | Equity securities | Common equities | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | | |
Available-for-sale Securities | 0.3 | 0.4 | 0.3 |
Fair Value, Inputs, Level 3 | Equity securities | Common equities | Common equities | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | | |
Available-for-sale Securities | 0 | 0 | 0 |
Fair Value, Inputs, Level 3 | Equity securities | Common equities | Other risk investments | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | | |
Available-for-sale Securities | 0.3 | 0.4 | 0.3 |
Fair Value, Inputs, Level 3 | Total Fixed Maturities and Equity Securities | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | | |
Available-for-sale Securities | 0.3 | 0.7 | 9.5 |
Fair Value, Inputs, Level 3 | Short-term investments | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | | |
Available-for-sale Securities | $ 0 | $ 0 | $ 0 |