Investments - Schedule of Investments (Details) (USD $)	Mar. 31, 2014	Dec. 31, 2013
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Debt Securities, Amortized Cost Basis	$ 4,312,700,000
Available-for-sale Debt Securities, Fair Value	4,619,500,000	4,575,000,000
Available for sale Securities, Equity Securities, Cost	562,100,000	530,000,000
Available for sale Securities, Equity Securities, Fair Value	648,300,000	598,500,000
Available-for-sale Securities, Debt Maturities, Fair Value [Abstract]
Due in One Year or Less - Amortized Cost	59,700,000
Due in One Year or Less - Fair Value	61,300,000
Due after One Year to Five Years - Amortized Cost	731,600,000
Due after One Year to Five Years - Fair Value	775,700,000
Due after Five Years to Ten Years - Amortized Cost	1,296,300,000
Due after Five Years to Ten Years - Fair Value	1,339,600,000
Due after Ten Years - Amortized Cost	2,023,600,000
Due after Ten Years - Fair Value	2,230,400,000
Asset-backed Securities Not Due in a Single Maturity Date - Amortized Cost	201,500,000
Asset-backed Securities Not Due in a Single Maturity Date - Fair Value	212,500,000
Other Receivables	225,500,000	193,100,000
Other Liabilities	367,700,000	329,800,000
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Less Than 12 Months, Fair Value	1,067,400,000	1,199,100,000
Less Than 12 Months, Unrealized Losses	(45,800,000)	(76,000,000)
12 Months or Longer, Fair Value	136,900,000	119,600,000
12 Months or Longer, Unrealized Losses	(8,400,000)	(9,900,000)
Total, Fair Value	1,204,300,000	1,318,700,000
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Losses Accumulated in Investments	(54,200,000)	(85,900,000)
Less Than 12 Months, Fair Value	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	200,000	300,000
Fixed Maturities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Debt Securities, Amortized Cost Basis	4,312,700,000	4,370,500,000
Available-for-sale Debt Securities Gross Unrealized Gain	356,200,000	284,200,000
Available-for-sale Debt Securities, Gross Unrealized Loss	(49,400,000)	(79,700,000)
Available-for-sale Debt Securities, Fair Value	4,619,500,000	4,575,000,000
Available-for-sale Securities, Debt Maturities, Fair Value [Abstract]
Other Receivables	500,000	2,500,000
Other Liabilities	33,600,000	0
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Less Than 12 Months, Fair Value	925,000,000	1,032,400,000
Less Than 12 Months, Unrealized Losses	(41,500,000)	(70,500,000)
12 Months or Longer, Fair Value	131,300,000	110,700,000
12 Months or Longer, Unrealized Losses	(7,900,000)	(9,200,000)
Total, Fair Value	1,056,300,000	1,143,100,000
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Losses Accumulated in Investments	(49,400,000)	(79,700,000)
US Government and Government Agencies and Authorities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Debt Securities, Amortized Cost Basis	339,300,000	351,100,000
Available-for-sale Debt Securities Gross Unrealized Gain	26,200,000	22,800,000
Available-for-sale Debt Securities, Gross Unrealized Loss	(8,200,000)	(11,700,000)
Available-for-sale Debt Securities, Fair Value	357,300,000	362,200,000
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Less Than 12 Months, Fair Value	86,900,000	95,800,000
Less Than 12 Months, Unrealized Losses	(7,600,000)	(10,900,000)
12 Months or Longer, Fair Value	4,600,000	4,400,000
12 Months or Longer, Unrealized Losses	(600,000)	(800,000)
Total, Fair Value	91,500,000	100,200,000
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Losses Accumulated in Investments	(8,200,000)	(11,700,000)
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	151,100,000
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	10,600,000
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	400,000
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	50,400,000
States and Political Subdivisions [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Debt Securities, Amortized Cost Basis	1,296,700,000	1,327,400,000
Available-for-sale Debt Securities Gross Unrealized Gain	79,000,000	53,800,000
Available-for-sale Debt Securities, Gross Unrealized Loss	(11,600,000)	(20,200,000)
Available-for-sale Debt Securities, Fair Value	1,364,100,000	1,361,000,000
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Less Than 12 Months, Fair Value	213,000,000	222,900,000
Less Than 12 Months, Unrealized Losses	(11,500,000)	(20,100,000)
12 Months or Longer, Fair Value	4,400,000	2,000,000
12 Months or Longer, Unrealized Losses	(100,000)	(100,000)
Total, Fair Value	217,400,000	224,900,000
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Losses Accumulated in Investments	(11,600,000)	(20,200,000)
Bonds and Notes [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Debt Securities, Amortized Cost Basis	2,621,900,000	2,636,400,000
Available-for-sale Debt Securities Gross Unrealized Gain	247,800,000	205,000,000
Available-for-sale Debt Securities, Gross Unrealized Loss	(29,500,000)	(47,700,000)
Available-for-sale Debt Securities, Fair Value	2,840,200,000	2,793,700,000
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Less Than 12 Months, Fair Value	617,300,000	699,800,000
Less Than 12 Months, Unrealized Losses	(22,400,000)	(39,400,000)
12 Months or Longer, Fair Value	119,500,000	103,200,000
12 Months or Longer, Unrealized Losses	(7,100,000)	(8,300,000)
Total, Fair Value	736,800,000	803,000,000
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Losses Accumulated in Investments	(29,500,000)	(47,700,000)
Redeemable Preferred Stocks [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Debt Securities, Amortized Cost Basis	6,500,000	6,600,000
Available-for-sale Debt Securities Gross Unrealized Gain	1,000,000	800,000
Available-for-sale Debt Securities, Gross Unrealized Loss	0	0
Available-for-sale Debt Securities, Fair Value	7,500,000	7,400,000
Collateralized Loan Obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Debt Securities, Amortized Cost Basis	44,200,000	44,200,000
Available-for-sale Debt Securities Gross Unrealized Gain	800,000	500,000
Available-for-sale Debt Securities, Gross Unrealized Loss	0	0
Available-for-sale Debt Securities, Fair Value	45,000,000	44,700,000
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Less Than 12 Months, Fair Value	7,500,000
Less Than 12 Months, Unrealized Losses	0
12 Months or Longer, Fair Value	2,300,000
12 Months or Longer, Unrealized Losses	0
Total, Fair Value	9,800,000
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Losses Accumulated in Investments	0
Mortgage and Asset-backed [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Debt Securities, Amortized Cost Basis	4,100,000	4,800,000
Available-for-sale Debt Securities Gross Unrealized Gain	1,400,000	1,300,000
Available-for-sale Debt Securities, Gross Unrealized Loss	(100,000)	(100,000)
Available-for-sale Debt Securities, Fair Value	5,400,000	6,000,000
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Less Than 12 Months, Fair Value	300,000	13,900,000
Less Than 12 Months, Unrealized Losses	0	(100,000)
12 Months or Longer, Fair Value	500,000	1,100,000
12 Months or Longer, Unrealized Losses	(100,000)	0
Total, Fair Value	800,000	15,000,000
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Losses Accumulated in Investments	(100,000)	(100,000)
Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available for sale Securities, Equity Securities, Cost	562,100,000	530,000,000
Available-for-sale Equity Securities, Gross Unrealized Gain	91,000,000	74,700,000
Available-for-sale Equity Securities, Gross Unrealized Loss	4,800,000	6,200,000
Available for sale Securities, Equity Securities, Fair Value	648,300,000	598,500,000
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Less Than 12 Months, Fair Value	142,400,000	166,700,000
Less Than 12 Months, Unrealized Losses	(4,300,000)	(5,500,000)
12 Months or Longer, Fair Value	5,600,000	8,900,000
12 Months or Longer, Unrealized Losses	(500,000)	(700,000)
Total, Fair Value	148,000,000	175,600,000
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Losses Accumulated in Investments	(4,800,000)	(6,200,000)
Exchange Traded Funds [Member]
Schedule of Available-for-sale Securities [Line Items]
Available for sale Securities, Equity Securities, Cost	143,600,000	122,000,000
Available-for-sale Equity Securities, Gross Unrealized Gain	7,500,000	3,900,000
Available-for-sale Equity Securities, Gross Unrealized Loss	400,000	1,000,000
Available for sale Securities, Equity Securities, Fair Value	150,700,000	124,900,000
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Less Than 12 Months, Fair Value	39,300,000	67,600,000
Less Than 12 Months, Unrealized Losses	(400,000)	(1,000,000)
12 Months or Longer, Fair Value	0	0
12 Months or Longer, Unrealized Losses	0	0
Total, Fair Value	39,300,000	67,600,000
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Losses Accumulated in Investments	(400,000)	(1,000,000)
Limited Liability Companies and Limited Partnerships [Member]
Schedule of Available-for-sale Securities [Line Items]
Available for sale Securities, Equity Securities, Cost	156,700,000	150,300,000
Available-for-sale Equity Securities, Gross Unrealized Gain	31,600,000	25,200,000
Available-for-sale Equity Securities, Gross Unrealized Loss	1,000,000	1,600,000
Available for sale Securities, Equity Securities, Fair Value	187,300,000	173,900,000
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Less Than 12 Months, Fair Value	20,800,000	53,100,000
Less Than 12 Months, Unrealized Losses	(700,000)	(900,000)
12 Months or Longer, Fair Value	3,900,000	5,000,000
12 Months or Longer, Unrealized Losses	(300,000)	(700,000)
Total, Fair Value	24,700,000	58,100,000
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Losses Accumulated in Investments	(1,000,000)	(1,600,000)
External Credit Rating, Investment Grade [Member] | Fixed Maturities [Member]
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Losses Accumulated in Investments	(44,800,000)	(74,200,000)
External Credit Rating, Non Investment Grade [Member] | Fixed Maturities [Member]
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Losses Accumulated in Investments	(4,600,000)	(5,500,000)
Percentage of Unrealized Loss Postion to Amortized Cost Basis of Available for Sale Security Average	5.00%	5.00%
Finance, Insurance and Real Estate [Member] | Preferred Stock [Member]
Schedule of Available-for-sale Securities [Line Items]
Available for sale Securities, Equity Securities, Cost	85,400,000	85,400,000
Available-for-sale Equity Securities, Gross Unrealized Gain	4,400,000	2,900,000
Available-for-sale Equity Securities, Gross Unrealized Loss	1,300,000	2,500,000
Available for sale Securities, Equity Securities, Fair Value	88,500,000	85,800,000
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Less Than 12 Months, Fair Value	29,200,000	22,500,000
Less Than 12 Months, Unrealized Losses	(1,300,000)	(2,500,000)
12 Months or Longer, Fair Value	0	2,500,000
12 Months or Longer, Unrealized Losses	0	0
Total, Fair Value	29,200,000	25,000,000
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Losses Accumulated in Investments	(1,300,000)	(2,500,000)
All Industries Other than Finance, Insurance and Real Estate [Member] | Preferred Stock [Member]
Schedule of Available-for-sale Securities [Line Items]
Available for sale Securities, Equity Securities, Cost	21,100,000	20,100,000
Available-for-sale Equity Securities, Gross Unrealized Gain	5,400,000	4,400,000
Available-for-sale Equity Securities, Gross Unrealized Loss	100,000	100,000
Available for sale Securities, Equity Securities, Fair Value	26,400,000	24,400,000
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Less Than 12 Months, Fair Value	4,300,000	4,300,000
Less Than 12 Months, Unrealized Losses	(100,000)	(100,000)
12 Months or Longer, Fair Value	800,000	700,000
12 Months or Longer, Unrealized Losses	0	0
Total, Fair Value	5,100,000	5,000,000
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Losses Accumulated in Investments	(100,000)	(100,000)
2000 to 3999 Manufacturing [Member] | Common Stocks [Member]
Schedule of Available-for-sale Securities [Line Items]
Available for sale Securities, Equity Securities, Cost	83,700,000	83,400,000
Available-for-sale Equity Securities, Gross Unrealized Gain	23,500,000	21,300,000
Available-for-sale Equity Securities, Gross Unrealized Loss	1,400,000	100,000
Available for sale Securities, Equity Securities, Fair Value	105,800,000	104,600,000
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Less Than 12 Months, Fair Value	33,300,000	5,000,000
Less Than 12 Months, Unrealized Losses	(1,200,000)	(100,000)
12 Months or Longer, Fair Value	500,000	200,000
12 Months or Longer, Unrealized Losses	(200,000)	0
Total, Fair Value	33,800,000	5,200,000
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Losses Accumulated in Investments	(1,400,000)	(100,000)
All Industries Other than Manufacturing [Member] | Common Stocks [Member]
Schedule of Available-for-sale Securities [Line Items]
Available for sale Securities, Equity Securities, Cost	71,600,000	68,800,000
Available-for-sale Equity Securities, Gross Unrealized Gain	18,600,000	17,000,000
Available-for-sale Equity Securities, Gross Unrealized Loss	600,000	900,000
Available for sale Securities, Equity Securities, Fair Value	89,600,000	84,900,000
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Less Than 12 Months, Fair Value	15,500,000	14,200,000
Less Than 12 Months, Unrealized Losses	(600,000)	(900,000)
12 Months or Longer, Fair Value	400,000	500,000
12 Months or Longer, Unrealized Losses	0	0
Total, Fair Value	15,900,000	14,700,000
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Losses Accumulated in Investments	$ (600,000)	$ (900,000)