Investments - Schedule of Investments (Details) - USD ($)	Sep. 30, 2017	Dec. 31, 2016
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)