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Investments - Schedule of Investments (Details) - USD ($) | Sep. 30, 2017 | Dec. 31, 2016
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Schedule of Available-for-sale Securities [Line Items] |  |
Available-for-sale Debt Securities, Amortized Cost Basis | $ 4,876,700,000 | $ 4,846,800,000
Fixed Maturities at Fair Value (Amortized Cost: 2017 - $4,876.7; 2016 - $4,846.8) | 5,242,800,000 | 5,124,900,000
Available for sale Securities, Equity Securities, Cost | 469,100,000 | 434,400,000
Available for sale Securities, Equity Securities, Fair Value | 512,100,000 | 481,700,000
Available-for-sale Securities, Debt Maturities, Fair Value [Abstract] |  |
Due in One Year or Less - Amortized Cost | 188,300,000 |
Due in One Year or Less - Fair Value | 190,800,000 |
Due after One Year to Five Years - Amortized Cost | 785,900,000 |
Due after One Year to Five Years - Fair Value | 814,300,000 |
Due after Five Years to Ten Years - Amortized Cost | 1,632,700,000 |
Due after Five Years to Ten Years - Fair Value | 1,710,900,000 |
Due after Ten Years - Amortized Cost | 1,988,100,000 |
Due after Ten Years - Fair Value | 2,234,300,000 |
Asset-backed Securities Not Due in a Single Maturity Date - Amortized Cost | 281,700,000 |
Asset-backed Securities Not Due in a Single Maturity Date - Fair Value | 292,500,000 |
Other Receivables | 182,400,000 | 198,600,000
Other Liabilities | 457,400,000 | 453,200,000
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract] |  |
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value | 580,200,000 | 1,273,900,000
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss | (13,800,000) | (40,500,000)
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value | 201,700,000 | 190,300,000
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss | (11,400,000) | (11,600,000)
Total, Fair Value | 781,900,000 | 1,464,200,000
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Losses Accumulated in Investments | (25,200,000) | (52,100,000)
Available-for-sale-Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Securities that have had Recognized Credit Losses in Earnings, Aggregate Losses | 0 | 100,000
Available For Sale Securities, Continuous Unrealized Loss Position, Credit Losses Recognized Related to Securities in An Unrealized Loss Position, In a Position of 12 Months or Longer | 0 | 0
US Government and Government Agencies and Authorities [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Available-for-sale Debt Securities, Amortized Cost Basis | 342,800,000 | 321,200,000
Available-for-sale Debt Securities Gross Unrealized Gain | 21,200,000 | 22,300,000
Available-for-sale Debt Securities, Gross Unrealized Loss | (4,900,000) | (7,200,000)
Fixed Maturities at Fair Value (Amortized Cost: 2017 - $4,876.7; 2016 - $4,846.8) | 359,100,000 | 336,300,000
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract] |  |
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value | 67,400,000 | 117,700,000
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss | (2,100,000) | (7,200,000)
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value | 41,200,000 | 1,100,000
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss | (2,800,000) | 0
Total, Fair Value | 108,600,000 | 118,800,000
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Losses Accumulated in Investments | (4,900,000) | (7,200,000)
Government National Mortgage Association Certificates and Obligations (GNMA) [Member] |  |
Available-for-sale Securities, Debt Maturities, Fair Value [Abstract] |  |
Asset-backed Securities Not Due in a Single Maturity Date - Fair Value | 152,100,000 |
Federal National Mortgage Association Certificates and Obligations (FNMA) [Member] |  |
Available-for-sale Securities, Debt Maturities, Fair Value [Abstract] |  |
Asset-backed Securities Not Due in a Single Maturity Date - Fair Value | 11,200,000 |
Federal Home Loan Mortgage Corporation Certificates and Obligations (FHLMC) [Member] |  |
Available-for-sale Securities, Debt Maturities, Fair Value [Abstract] |  |
Asset-backed Securities Not Due in a Single Maturity Date - Fair Value | 4,100,000 |
States and Political Subdivisions [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Available-for-sale Debt Securities, Amortized Cost Basis | 1,615,000,000 | 1,640,600,000
Available-for-sale Debt Securities Gross Unrealized Gain | 110,800,000 | 88,400,000
Available-for-sale Debt Securities, Gross Unrealized Loss | (2,800,000) | (14,100,000)
Fixed Maturities at Fair Value (Amortized Cost: 2017 - $4,876.7; 2016 - $4,846.8) | 1,723,000,000 | 1,714,900,000
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract] |  |
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value | 169,300,000 | 432,700,000
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss | (2,100,000) | (14,100,000)
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value | 28,900,000 | 300,000
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss | (700,000) | 0
Total, Fair Value | 198,200,000 | 433,000,000
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Losses Accumulated in Investments | (2,800,000) | (14,100,000)
Foreign Government Debt Securities [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Available-for-sale Debt Securities, Amortized Cost Basis | 3,100,000 | 3,500,000
Available-for-sale Debt Securities Gross Unrealized Gain | 200,000 | 0
Available-for-sale Debt Securities, Gross Unrealized Loss | 0 | (100,000)
Fixed Maturities at Fair Value (Amortized Cost: 2017 - $4,876.7; 2016 - $4,846.8) | 3,300,000 | 3,400,000
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract] |  |
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value |  | 2,100,000
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss |  | (100,000)
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value |  | 0
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss |  | 0
Total, Fair Value |  | 2,100,000
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Losses Accumulated in Investments |  | (100,000)
Corporate Bonds and Notes [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Available-for-sale Debt Securities, Amortized Cost Basis | 2,796,900,000 | 2,758,900,000
Available-for-sale Debt Securities Gross Unrealized Gain | 247,200,000 | 209,900,000
Available-for-sale Debt Securities, Gross Unrealized Loss | (11,000,000) | (24,000,000)
Fixed Maturities at Fair Value (Amortized Cost: 2017 - $4,876.7; 2016 - $4,846.8) | 3,033,100,000 | 2,944,800,000
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract] |  |
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value | 292,700,000 | 663,300,000
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss | (6,700,000) | (16,600,000)
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value | 96,200,000 | 107,300,000
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss | (4,300,000) | (7,400,000)
Total, Fair Value | 388,900,000 | 770,600,000
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Losses Accumulated in Investments | (11,000,000) | (24,000,000)
Redeemable Preferred Stocks [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Available-for-sale Debt Securities, Amortized Cost Basis | 100,000 | 500,000
Available-for-sale Debt Securities Gross Unrealized Gain | 0 | 100,000
Available-for-sale Debt Securities, Gross Unrealized Loss | 0 | 0
Fixed Maturities at Fair Value (Amortized Cost: 2017 - $4,876.7; 2016 - $4,846.8) | 100,000 | 600,000
Collateralized Loan Obligations [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Available-for-sale Debt Securities, Amortized Cost Basis | 118,400,000 | 121,200,000
Available-for-sale Debt Securities Gross Unrealized Gain | 4,700,000 | 2,700,000
Available-for-sale Debt Securities, Gross Unrealized Loss | 0 | (1,100,000)
Fixed Maturities at Fair Value (Amortized Cost: 2017 - $4,876.7; 2016 - $4,846.8) | 123,100,000 | 122,800,000
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract] |  |
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value | 2,000,000 | 19,900,000
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss | 0 | (700,000)
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value | 2,400,000 | 21,400,000
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss | 0 | (400,000)
Total, Fair Value | 4,400,000 | 41,300,000
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Losses Accumulated in Investments | 0 | (1,100,000)
Other Mortgage- and Asset-backed [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Available-for-sale Debt Securities, Amortized Cost Basis | 400,000 | 900,000
Available-for-sale Debt Securities Gross Unrealized Gain | 700,000 | 1,200,000
Available-for-sale Debt Securities, Gross Unrealized Loss | 0 | 0
Fixed Maturities at Fair Value (Amortized Cost: 2017 - $4,876.7; 2016 - $4,846.8) | 1,100,000 | 2,100,000
Fixed Maturities [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Available-for-sale Debt Securities, Amortized Cost Basis | 4,876,700,000 | 4,846,800,000
Available-for-sale Debt Securities Gross Unrealized Gain | 384,800,000 | 324,600,000
Available-for-sale Debt Securities, Gross Unrealized Loss | (18,700,000) | (46,500,000)
Fixed Maturities at Fair Value (Amortized Cost: 2017 - $4,876.7; 2016 - $4,846.8) | 5,242,800,000 | 5,124,900,000
Available-for-sale Securities, Debt Maturities, Fair Value [Abstract] |  |
Other Receivables | 0 | 2,700,000
Other Liabilities | 4,700,000 | 100,000
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract] |  |
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value | 531,400,000 | 1,235,700,000
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss | (10,900,000) | (38,700,000)
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value | 168,700,000 | 130,100,000
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss | (7,800,000) | (7,800,000)
Total, Fair Value | 700,100,000 | 1,365,800,000
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Losses Accumulated in Investments | (18,700,000) | (46,500,000)
Fixed Maturities [Member] | External Credit Rating, Investment Grade [Member] |  |
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract] |  |
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Losses Accumulated in Investments | (12,000,000) | (33,800,000)
Fixed Maturities [Member] | External Credit Rating, Non Investment Grade [Member] |  |
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract] |  |
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Losses Accumulated in Investments | $ (6,700,000) | $ (12,700,000)
Percentage of Unrealized Loss Position to Amortized Cost Basis of Available for Sale Security Average | 4.00% | 5.00%
Other Non-Governmental Issuers [Member] |  |
Available-for-sale Securities, Debt Maturities, Fair Value [Abstract] |  |
Asset-backed Securities Not Due in a Single Maturity Date - Fair Value | $ 125,100,000 |
Equity Securities [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Available for sale Securities, Equity Securities, Cost | 469,100,000 | $ 434,400,000
Available-for-sale Equity Securities, Gross Unrealized Gain | 49,500,000 | 52,900,000
Available-for-sale Equity Securities, Gross Unrealized Loss | (6,500,000) | (5,600,000)
Available for sale Securities, Equity Securities, Fair Value | 512,100,000 | 481,700,000
Available-for-sale Securities, Debt Maturities, Fair Value [Abstract] |  |
Other Receivables | 0 | 200,000
Other Liabilities | 0 | 0
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract] |  |
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value | 48,800,000 | 38,200,000
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss | (2,900,000) | (1,800,000)
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value | 33,000,000 | 60,200,000
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss | (3,600,000) | (3,800,000)
Total, Fair Value | 81,800,000 | 98,400,000
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Losses Accumulated in Investments | (6,500,000) | (5,600,000)
Preferred Stock [Member] | Finance, Insurance and Real Estate [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Available for sale Securities, Equity Securities, Cost | 53,600,000 | 58,100,000
Available-for-sale Equity Securities, Gross Unrealized Gain | 5,000,000 | 2,300,000
Available-for-sale Equity Securities, Gross Unrealized Loss | 0 | (800,000)
Available for sale Securities, Equity Securities, Fair Value | 58,600,000 | 59,600,000
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract] |  |
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value | 0 | 15,600,000
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss | 0 | (500,000)
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value | 200,000 | 7,300,000
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss | 0 | (300,000)
Total, Fair Value | 200,000 | 22,900,000
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Losses Accumulated in Investments | 0 | (800,000)
Preferred Stock [Member] | All Industries Other than Finance, Insurance and Real Estate [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Available for sale Securities, Equity Securities, Cost | 18,400,000 | 18,500,000
Available-for-sale Equity Securities, Gross Unrealized Gain | 6,200,000 | 4,900,000
Available-for-sale Equity Securities, Gross Unrealized Loss | 0 | (500,000)
Available for sale Securities, Equity Securities, Fair Value | 24,600,000 | 22,900,000
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract] |  |
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value | 100,000 | 5,300,000
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss | 0 | (500,000)
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value | 200,000 | 0
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss | 0 | 0
Total, Fair Value | 300,000 | 5,300,000
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Losses Accumulated in Investments | 0 | (500,000)
Common Stocks [Member] | Finance, Insurance and Real Estate [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Available for sale Securities, Equity Securities, Cost | 6,700,000 | 31,200,000
Available-for-sale Equity Securities, Gross Unrealized Gain | 1,300,000 | 2,300,000
Available-for-sale Equity Securities, Gross Unrealized Loss | (800,000) | 0
Available for sale Securities, Equity Securities, Fair Value | 7,200,000 | 33,500,000
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract] |  |
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value | 2,200,000 | 2,800,000
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss | (800,000) | 0
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value | 0 | 0
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss | 0 | 0
Total, Fair Value | 2,200,000 | 2,800,000
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Losses Accumulated in Investments | (800,000) | 0
Common Stocks [Member] | All Industries Other than Finance, Insurance and Real Estate [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Available for sale Securities, Equity Securities, Cost | 8,900,000 | 7,200,000
Available-for-sale Equity Securities, Gross Unrealized Gain | 5,100,000 | 4,600,000
Available-for-sale Equity Securities, Gross Unrealized Loss | (200,000) | (100,000)
Available for sale Securities, Equity Securities, Fair Value | 13,800,000 | 11,700,000
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract] |  |
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value | 200,000 | 600,000
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss | (200,000) | (100,000)
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value | 500,000 | 500,000
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss | 0 | 0
Total, Fair Value | 700,000 | 1,100,000
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Losses Accumulated in Investments | (200,000) | (100,000)
Exchange Traded Funds [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Available for sale Securities, Equity Securities, Cost | 196,700,000 | 136,100,000
Available-for-sale Equity Securities, Gross Unrealized Gain | 8,300,000 | 9,600,000
Available-for-sale Equity Securities, Gross Unrealized Loss | 0 | (1,300,000)
Available for sale Securities, Equity Securities, Fair Value | 205,000,000 | 144,400,000
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract] |  |
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value |  | 0
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss |  | 0
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value |  | 18,600,000
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss |  | (1,300,000)
Total, Fair Value |  | 18,600,000
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Losses Accumulated in Investments |  | (1,300,000)
Limited Liability Companies and Limited Partnerships [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Available for sale Securities, Equity Securities, Cost | 184,800,000 | 183,300,000
Available-for-sale Equity Securities, Gross Unrealized Gain | 23,600,000 | 29,200,000
Available-for-sale Equity Securities, Gross Unrealized Loss | (5,500,000) | (2,900,000)
Available for sale Securities, Equity Securities, Fair Value | 202,900,000 | 209,600,000
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract] |  |
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value | 46,300,000 | 13,900,000
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss | (1,900,000) | (700,000)
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value | 32,100,000 | 33,800,000
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss | (3,600,000) | (2,200,000)
Total, Fair Value | 78,400,000 | 47,700,000
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Losses Accumulated in Investments | $ (5,500,000) | $ (2,900,000)

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- Definition This item represents the cost of debt securities, which are categorized neither as held-to-maturity nor trading, net of adjustments including accretion, amortization, collection of cash, previous other-than-temporary impairments recognized in earnings (less any cumulative-effect adjustments recognized, as defined), and fair value hedge accounting adjustments, if any. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (aa) -URI http://asc.fasb.org/extlink&oid=73527792&loc=d3e27161-111563 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.2) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 + Details
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