Investments - Schedule of Investments (Details) - USD ($)	Sep. 30, 2015	Dec. 31, 2014
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Debt Securities, Amortized Cost Basis	$ 4,496,000,000	$ 4,341,700,000
Available-for-sale Debt Securities, Fair Value	4,844,900,000	4,777,600,000
Available for sale Securities, Equity Securities, Cost	509,800,000	561,500,000
Available for sale Securities, Equity Securities, Fair Value	535,700,000	632,200,000
Available-for-sale Securities, Debt Maturities, Fair Value [Abstract]
Due in One Year or Less - Amortized Cost	58,900,000
Due in One Year or Less - Fair Value	59,800,000
Due after One Year to Five Years - Amortized Cost	793,800,000
Due after One Year to Five Years - Fair Value	837,600,000
Due after Five Years to Ten Years - Amortized Cost	1,414,000,000
Due after Five Years to Ten Years - Fair Value	1,459,400,000
Due after Ten Years - Amortized Cost	2,021,000,000
Due after Ten Years - Fair Value	2,270,800,000
Asset-backed Securities Not Due in a Single Maturity Date - Amortized Cost	208,300,000
Asset-backed Securities Not Due in a Single Maturity Date - Fair Value	217,300,000
Other Receivables	198,900,000	187,000,000
Other Liabilities	486,800,000	409,600,000
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	1,138,700,000	423,600,000
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(38,300,000)	(9,700,000)
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	211,500,000	580,700,000
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(12,000,000)	(15,000,000)
Total, Fair Value	1,350,200,000	1,004,300,000
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Losses Accumulated in Investments	(50,300,000)	(24,700,000)
Available-for-sale-Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Securities that have had Recognized Credit Losses in Earnings, Aggregate Losses	0	0
Available For Sale Securities, Continuous Unrealized Loss Position, Credit Losses Recognized Related to Securites in An Unrealized Loss Position, In a Position of 12 Months or Longer	0	0
Fixed Maturities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Debt Securities, Amortized Cost Basis	4,496,000,000	4,341,700,000
Available-for-sale Debt Securities Gross Unrealized Gain	381,500,000	455,600,000
Available-for-sale Debt Securities, Gross Unrealized Loss	(32,600,000)	(19,700,000)
Available-for-sale Debt Securities, Fair Value	4,844,900,000	4,777,600,000
Available-for-sale Securities, Debt Maturities, Fair Value [Abstract]
Other Receivables	0	0
Other Liabilities	12,400,000	0
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	886,500,000	327,500,000
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(23,000,000)	(6,500,000)
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	146,500,000	546,100,000
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(9,600,000)	(13,200,000)
Total, Fair Value	1,033,000,000	873,600,000
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Losses Accumulated in Investments	(32,600,000)	(19,700,000)
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	(20,400,000)	(14,100,000)
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	$ (12,200,000)	$ (5,600,000)
Percentage of Unrealized Loss Postion to Amortized Cost Basis of Available for Sale Security Average	6.00%	4.00%
US Government and Government Agencies and Authorities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Debt Securities, Amortized Cost Basis	$ 303,600,000	$ 315,200,000
Available-for-sale Debt Securities Gross Unrealized Gain	29,900,000	32,300,000
Available-for-sale Debt Securities, Gross Unrealized Loss	(2,200,000)	(2,000,000)
Available-for-sale Debt Securities, Fair Value	331,300,000	345,500,000
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	75,600,000	24,900,000
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(900,000)	(700,000)
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	24,700,000	55,500,000
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(1,300,000)	(1,300,000)
Total, Fair Value	100,300,000	80,400,000
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Losses Accumulated in Investments	(2,200,000)	(2,000,000)
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	113,200,000
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	18,200,000
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	6,300,000
States and Political Subdivisions [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Debt Securities, Amortized Cost Basis	1,482,300,000	1,352,500,000
Available-for-sale Debt Securities Gross Unrealized Gain	111,400,000	126,400,000
Available-for-sale Debt Securities, Gross Unrealized Loss	(2,000,000)	(1,800,000)
Available-for-sale Debt Securities, Fair Value	1,591,700,000	1,477,100,000
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	160,700,000	1,000,000
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(1,900,000)	0
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	900,000	126,300,000
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(100,000)	(1,800,000)
Total, Fair Value	161,600,000	127,300,000
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Losses Accumulated in Investments	(2,000,000)	(1,800,000)
Bonds and Notes [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Debt Securities, Amortized Cost Basis	2,626,800,000	2,599,300,000
Available-for-sale Debt Securities Gross Unrealized Gain	238,200,000	294,300,000
Available-for-sale Debt Securities, Gross Unrealized Loss	(27,200,000)	(15,100,000)
Available-for-sale Debt Securities, Fair Value	2,837,800,000	2,878,500,000
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	605,900,000	250,400,000
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(19,100,000)	(5,100,000)
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	119,800,000	360,500,000
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(8,100,000)	(10,000,000)
Total, Fair Value	725,700,000	610,900,000
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Losses Accumulated in Investments	(27,200,000)	(15,100,000)
Redeemable Preferred Stocks [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Debt Securities, Amortized Cost Basis	4,400,000	5,900,000
Available-for-sale Debt Securities Gross Unrealized Gain	100,000	800,000
Available-for-sale Debt Securities, Gross Unrealized Loss	0	0
Available-for-sale Debt Securities, Fair Value	4,500,000	6,700,000
Collateralized Loan Obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Debt Securities, Amortized Cost Basis	75,100,000	64,900,000
Available-for-sale Debt Securities Gross Unrealized Gain	400,000	300,000
Available-for-sale Debt Securities, Gross Unrealized Loss	(1,200,000)	(800,000)
Available-for-sale Debt Securities, Fair Value	74,300,000	64,400,000
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	44,300,000	51,200,000
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(1,100,000)	(700,000)
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	800,000	3,400,000
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(100,000)	(100,000)
Total, Fair Value	45,100,000	54,600,000
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Losses Accumulated in Investments	(1,200,000)	(800,000)
Other Mortgage- and Asset-backed [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Debt Securities, Amortized Cost Basis	3,800,000	3,900,000
Available-for-sale Debt Securities Gross Unrealized Gain	1,500,000	1,500,000
Available-for-sale Debt Securities, Gross Unrealized Loss	0	0
Available-for-sale Debt Securities, Fair Value	5,300,000	5,400,000
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	0	0
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	0	0
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	300,000	400,000
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	0	0
Total, Fair Value	300,000	400,000
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Losses Accumulated in Investments	0	0
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	79,600,000
Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available for sale Securities, Equity Securities, Cost	509,800,000	561,500,000
Available-for-sale Equity Securities, Gross Unrealized Gain	43,600,000	75,700,000
Available-for-sale Equity Securities, Gross Unrealized Loss	(17,700,000)	(5,000,000)
Available for sale Securities, Equity Securities, Fair Value	535,700,000	632,200,000
Available-for-sale Securities, Debt Maturities, Fair Value [Abstract]
Other Receivables	0	0
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	252,200,000	96,100,000
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(15,300,000)	(3,200,000)
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	65,000,000	34,600,000
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(2,400,000)	(1,800,000)
Total, Fair Value	317,200,000	130,700,000
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Losses Accumulated in Investments	(17,700,000)	(5,000,000)
Preferred Stock [Member] | Finance, Insurance and Real Estate [Member]
Schedule of Available-for-sale Securities [Line Items]
Available for sale Securities, Equity Securities, Cost	80,800,000	85,500,000
Available-for-sale Equity Securities, Gross Unrealized Gain	4,400,000	5,200,000
Available-for-sale Equity Securities, Gross Unrealized Loss	(900,000)	(1,000,000)
Available for sale Securities, Equity Securities, Fair Value	84,300,000	89,700,000
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	2,700,000	7,500,000
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	0	(100,000)
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	17,200,000	12,100,000
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(900,000)	(900,000)
Total, Fair Value	19,900,000	19,600,000
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Losses Accumulated in Investments	(900,000)	(1,000,000)
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	21,000,000	16,300,000
Available-for-sale Equity Securities, Gross Unrealized Gain	2,900,000	3,500,000
Available-for-sale Equity Securities, Gross Unrealized Loss	(500,000)	0
Available for sale Securities, Equity Securities, Fair Value	23,400,000	19,800,000
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	7,500,000	0
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(500,000)	0
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	8,000,000	500,000
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Losses Accumulated in Investments	(500,000)	0
Common Stocks [Member] | Finance, Insurance and Real Estate [Member]
Schedule of Available-for-sale Securities [Line Items]
Available for sale Securities, Equity Securities, Cost	18,400,000	34,300,000
Available-for-sale Equity Securities, Gross Unrealized Gain	4,100,000	6,200,000
Available-for-sale Equity Securities, Gross Unrealized Loss	(800,000)	0
Available for sale Securities, Equity Securities, Fair Value	21,700,000	40,500,000
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	9,900,000
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(800,000)
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	9,900,000
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Losses Accumulated in Investments	(800,000)
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	9,800,000
Available-for-sale Equity Securities, Gross Unrealized Gain	6,000,000
Available-for-sale Equity Securities, Gross Unrealized Loss	(500,000)
Available for sale Securities, Equity Securities, Fair Value	15,300,000
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	3,000,000
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(500,000)
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	3,000,000
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Losses Accumulated in Investments	(500,000)
Common Stocks [Member] | Manufacturing [Member]
Schedule of Available-for-sale Securities [Line Items]
Available for sale Securities, Equity Securities, Cost		43,400,000
Available-for-sale Equity Securities, Gross Unrealized Gain		14,900,000
Available-for-sale Equity Securities, Gross Unrealized Loss		(1,100,000)
Available for sale Securities, Equity Securities, Fair Value		57,200,000
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value		15,100,000
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss		(1,100,000)
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		15,100,000
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Losses Accumulated in Investments		(1,100,000)
Common Stocks [Member] | All Industries Other than Manufacturing and Finance, Insurance and Real Estate [Member]
Schedule of Available-for-sale Securities [Line Items]
Available for sale Securities, Equity Securities, Cost		26,500,000
Available-for-sale Equity Securities, Gross Unrealized Gain		10,000,000
Available-for-sale Equity Securities, Gross Unrealized Loss		(400,000)
Available for sale Securities, Equity Securities, Fair Value		36,100,000
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value		4,200,000
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss		(400,000)
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value		1,000,000
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss		0
Total, Fair Value		5,200,000
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Losses Accumulated in Investments		(400,000)
Exchange Traded Funds [Member]
Schedule of Available-for-sale Securities [Line Items]
Available for sale Securities, Equity Securities, Cost	198,700,000	195,200,000
Available-for-sale Equity Securities, Gross Unrealized Gain	300,000	8,200,000
Available-for-sale Equity Securities, Gross Unrealized Loss	(12,000,000)	(700,000)
Available for sale Securities, Equity Securities, Fair Value	187,000,000	202,700,000
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	178,200,000	14,900,000
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(12,000,000)	(100,000)
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	0	14,400,000
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	0	(600,000)
Total, Fair Value	178,200,000	29,300,000
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Losses Accumulated in Investments	(12,000,000)	(700,000)
Limited Liability Companies and Limited Partnerships [Member]
Schedule of Available-for-sale Securities [Line Items]
Available for sale Securities, Equity Securities, Cost	181,100,000	160,300,000
Available-for-sale Equity Securities, Gross Unrealized Gain	25,900,000	27,700,000
Available-for-sale Equity Securities, Gross Unrealized Loss	(3,000,000)	(1,800,000)
Available for sale Securities, Equity Securities, Fair Value	204,000,000	186,200,000
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	50,900,000	54,400,000
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(1,500,000)	(1,500,000)
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	47,300,000	6,600,000
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(1,500,000)	(300,000)
Total, Fair Value	98,200,000	61,000,000
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Losses Accumulated in Investments	$ (3,000,000)	$ (1,800,000)