Investments - Schedule of Investments (Details) - USD ($)	Jun. 30, 2015	Dec. 31, 2014
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Debt Securities, Amortized Cost Basis	$ 4,433,000,000	$ 4,341,700,000
Available-for-sale Debt Securities, Fair Value	4,764,000,000	4,777,600,000
Available for sale Securities, Equity Securities, Cost	518,800,000	561,500,000
Available for sale Securities, Equity Securities, Fair Value	562,900,000	632,200,000
Available-for-sale Securities, Debt Maturities, Fair Value [Abstract]
Due in One Year or Less - Amortized Cost	59,400,000
Due in One Year or Less - Fair Value	60,600,000
Due after One Year to Five Years - Amortized Cost	787,400,000
Due after One Year to Five Years - Fair Value	835,100,000
Due after Five Years to Ten Years - Amortized Cost	1,376,500,000
Due after Five Years to Ten Years - Fair Value	1,416,200,000
Due after Ten Years - Amortized Cost	1,998,500,000
Due after Ten Years - Fair Value	2,230,100,000
Asset-backed Securities Not Due in a Single Maturity Date - Amortized Cost	211,200,000
Asset-backed Securities Not Due in a Single Maturity Date - Fair Value	222,000,000
Other Receivables	267,100,000	187,000,000
Other Liabilities	495,600,000	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	1,236,900,000	423,600,000
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(30,000,000)	(9,700,000)
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	170,700,000	580,700,000
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(8,900,000)	(15,000,000)
Total, Fair Value	1,407,600,000	1,004,300,000
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Losses Accumulated in Investments	(38,900,000)	(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	(100,000)	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	4,433,000,000	4,341,700,000
Available-for-sale Debt Securities Gross Unrealized Gain	364,300,000	455,600,000
Available-for-sale Debt Securities, Gross Unrealized Loss	(33,300,000)	(19,700,000)
Available-for-sale Debt Securities, Fair Value	4,764,000,000	4,777,600,000
Available-for-sale Securities, Debt Maturities, Fair Value [Abstract]
Other Receivables	0	0
Other Liabilities	23,300,000	0
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	1,096,600,000	327,500,000
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(26,800,000)	(6,500,000)
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	125,400,000	546,100,000
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(6,500,000)	(13,200,000)
Total, Fair Value	1,222,000,000	873,600,000
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Losses Accumulated in Investments	(33,300,000)	(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	(26,800,000)	(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	$ (6,500,000)	$ (5,600,000)
Percentage of Unrealized Loss Postion to Amortized Cost Basis of Available for Sale Security Average	3.50%	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	$ 318,300,000	$ 315,200,000
Available-for-sale Debt Securities Gross Unrealized Gain	27,700,000	32,300,000
Available-for-sale Debt Securities, Gross Unrealized Loss	(4,800,000)	(2,000,000)
Available-for-sale Debt Securities, Fair Value	341,200,000	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	83,000,000	24,900,000
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(2,800,000)	(700,000)
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	24,000,000	55,500,000
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(2,000,000)	(1,300,000)
Total, Fair Value	107,000,000	80,400,000
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Losses Accumulated in Investments	(4,800,000)	(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	120,200,000
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	19,400,000
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	6,500,000
States and Political Subdivisions [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Debt Securities, Amortized Cost Basis	1,439,700,000	1,352,500,000
Available-for-sale Debt Securities Gross Unrealized Gain	97,500,000	126,400,000
Available-for-sale Debt Securities, Gross Unrealized Loss	(5,900,000)	(1,800,000)
Available-for-sale Debt Securities, Fair Value	1,531,300,000	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	355,700,000	1,000,000
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(5,800,000)	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	356,600,000	127,300,000
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Losses Accumulated in Investments	(5,900,000)	(1,800,000)
Bonds and Notes [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Debt Securities, Amortized Cost Basis	2,596,200,000	2,599,300,000
Available-for-sale Debt Securities Gross Unrealized Gain	236,400,000	294,300,000
Available-for-sale Debt Securities, Gross Unrealized Loss	(22,400,000)	(15,100,000)
Available-for-sale Debt Securities, Fair Value	2,810,200,000	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	638,700,000	250,400,000
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(18,000,000)	(5,100,000)
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	100,100,000	360,500,000
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(4,400,000)	(10,000,000)
Total, Fair Value	738,800,000	610,900,000
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Losses Accumulated in Investments	(22,400,000)	(15,100,000)
Redeemable Preferred Stocks [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Debt Securities, Amortized Cost Basis	5,300,000	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	5,400,000	6,700,000
Collateralized Loan Obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Debt Securities, Amortized Cost Basis	69,700,000	64,900,000
Available-for-sale Debt Securities Gross Unrealized Gain	900,000	300,000
Available-for-sale Debt Securities, Gross Unrealized Loss	(200,000)	(800,000)
Available-for-sale Debt Securities, Fair Value	70,400,000	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	19,200,000	51,200,000
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(200,000)	(700,000)
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	0	3,400,000
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	0	(100,000)
Total, Fair Value	19,200,000	54,600,000
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Losses Accumulated in Investments	(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	1,700,000	1,500,000
Available-for-sale Debt Securities, Gross Unrealized Loss	0	0
Available-for-sale Debt Securities, Fair Value	5,500,000	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	400,000	400,000
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	0	0
Total, Fair Value	400,000	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	75,900,000
Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available for sale Securities, Equity Securities, Cost	518,800,000	561,500,000
Available-for-sale Equity Securities, Gross Unrealized Gain	49,700,000	75,700,000
Available-for-sale Equity Securities, Gross Unrealized Loss	(5,600,000)	(5,000,000)
Available for sale Securities, Equity Securities, Fair Value	562,900,000	632,200,000
Available-for-sale Securities, Debt Maturities, Fair Value [Abstract]
Other Receivables	36,500,000	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	140,300,000	96,100,000
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(3,200,000)	(3,200,000)
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	45,300,000	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	185,600,000	130,700,000
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Losses Accumulated in Investments	(5,600,000)	(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	83,700,000	85,500,000
Available-for-sale Equity Securities, Gross Unrealized Gain	4,500,000	5,200,000
Available-for-sale Equity Securities, Gross Unrealized Loss	(800,000)	(1,000,000)
Available for sale Securities, Equity Securities, Fair Value	87,400,000	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	7,600,000	7,500,000
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	12,400,000	12,100,000
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(600,000)	(900,000)
Total, Fair Value	20,000,000	19,600,000
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Losses Accumulated in Investments	(800,000)	(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	21,300,000	16,300,000
Available-for-sale Equity Securities, Gross Unrealized Gain	3,500,000	3,500,000
Available-for-sale Equity Securities, Gross Unrealized Loss	(100,000)	0
Available for sale Securities, Equity Securities, Fair Value	24,700,000	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	900,000	0
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(100,000)	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	1,400,000	500,000
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Losses Accumulated in Investments	(100,000)	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	20,800,000	34,300,000
Available-for-sale Equity Securities, Gross Unrealized Gain	3,700,000	6,200,000
Available-for-sale Equity Securities, Gross Unrealized Loss	(900,000)	0
Available for sale Securities, Equity Securities, Fair Value	23,600,000	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	7,000,000	0
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(900,000)	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	7,000,000	0
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Losses Accumulated in Investments	(900,000)	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	9,800,000
Available-for-sale Equity Securities, Gross Unrealized Gain	5,400,000
Available-for-sale Equity Securities, Gross Unrealized Loss	0
Available for sale Securities, Equity Securities, Fair Value	15,200,000
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	800,000
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	0
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	0
Total, Fair Value	800,000
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Losses Accumulated in Investments	0
Common Stocks [Member] | 2000 to 3999 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	208,900,000	195,200,000
Available-for-sale Equity Securities, Gross Unrealized Gain	4,000,000	8,200,000
Available-for-sale Equity Securities, Gross Unrealized Loss	(800,000)	(700,000)
Available for sale Securities, Equity Securities, Fair Value	212,100,000	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	58,400,000	14,900,000
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(800,000)	(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	58,400,000	29,300,000
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Losses Accumulated in Investments	(800,000)	(700,000)
Limited Liability Companies and Limited Partnerships [Member]
Schedule of Available-for-sale Securities [Line Items]
Available for sale Securities, Equity Securities, Cost	174,300,000	160,300,000
Available-for-sale Equity Securities, Gross Unrealized Gain	28,600,000	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	199,900,000	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	65,600,000	54,400,000
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(1,200,000)	(1,500,000)
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	32,400,000	6,600,000
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(1,800,000)	(300,000)
Total, Fair Value	98,000,000	61,000,000
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Losses Accumulated in Investments	$ (3,000,000)	$ (1,800,000)