Investments - Schedule of Investments (Details) (USD $)	9 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Dec. 31, 2013
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Debt Securities, Amortized Cost Basis	$ 4,318,500,000		$ 4,370,500,000
Available-for-sale Debt Securities, Fair Value	4,697,200,000		4,575,000,000
Available for sale Securities, Equity Securities, Cost	547,300,000		530,000,000
Available for sale Securities, Equity Securities, Fair Value	661,400,000		598,500,000
Available-for-sale Securities, Debt Maturities, Fair Value [Abstract]
Due in One Year or Less - Amortized Cost	46,600,000
Due in One Year or Less - Fair Value	47,900,000
Due after One Year to Five Years - Amortized Cost	728,500,000
Due after One Year to Five Years - Fair Value	777,400,000
Due after Five Years to Ten Years - Amortized Cost	1,329,000,000
Due after Five Years to Ten Years - Fair Value	1,378,500,000
Due after Ten Years - Amortized Cost	2,009,500,000
Due after Ten Years - Fair Value	2,277,100,000
Asset-backed Securities Not Due in a Single Maturity Date - Amortized Cost	204,900,000
Asset-backed Securities Not Due in a Single Maturity Date - Fair Value	216,300,000
Other Receivables	247,500,000		193,100,000
Other Liabilities	390,300,000		329,800,000
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Less Than 12 Months, Fair Value	535,900,000		1,199,100,000
Less Than 12 Months, Unrealized Losses	(8,700,000)		(76,000,000)
12 Months or Longer, Fair Value	662,400,000		119,600,000
12 Months or Longer, Unrealized Losses	(25,600,000)		(9,900,000)
Total, Fair Value	1,198,300,000		1,318,700,000
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Losses Accumulated in Investments	(34,300,000)		(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	300,000		300,000
Purchases of Fair Value Option Investments	52,900,000	0
Sales of Fair Value Option Investments	2,900,000	0
Fair Value Option Investments	50,600,000		0
Fixed Maturities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Debt Securities, Amortized Cost Basis	4,318,500,000		4,370,500,000
Available-for-sale Debt Securities Gross Unrealized Gain	407,200,000		284,200,000
Available-for-sale Debt Securities, Gross Unrealized Loss	(28,500,000)		(79,700,000)
Available-for-sale Debt Securities, Fair Value	4,697,200,000		4,575,000,000
Available-for-sale Securities, Debt Maturities, Fair Value [Abstract]
Other Receivables	0		2,500,000
Other Liabilities	6,800,000		0
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Less Than 12 Months, Fair Value	361,400,000		1,032,400,000
Less Than 12 Months, Unrealized Losses	(4,600,000)		(70,500,000)
12 Months or Longer, Fair Value	632,400,000		110,700,000
12 Months or Longer, Unrealized Losses	(23,900,000)		(9,200,000)
Total, Fair Value	993,800,000		1,143,100,000
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Losses Accumulated in Investments	(28,500,000)		(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	320,400,000		351,100,000
Available-for-sale Debt Securities Gross Unrealized Gain	28,500,000		22,800,000
Available-for-sale Debt Securities, Gross Unrealized Loss	(5,400,000)		(11,700,000)
Available-for-sale Debt Securities, Fair Value	343,500,000		362,200,000
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Less Than 12 Months, Fair Value	24,300,000		95,800,000
Less Than 12 Months, Unrealized Losses	(1,400,000)		(10,900,000)
12 Months or Longer, Fair Value	65,300,000		4,400,000
12 Months or Longer, Unrealized Losses	(4,000,000)		(800,000)
Total, Fair Value	89,600,000		100,200,000
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Losses Accumulated in Investments	(5,400,000)		(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	138,300,000
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	9,600,000
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	300,000
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	68,100,000
States and Political Subdivisions [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Debt Securities, Amortized Cost Basis	1,336,000,000		1,327,400,000
Available-for-sale Debt Securities Gross Unrealized Gain	107,500,000		53,800,000
Available-for-sale Debt Securities, Gross Unrealized Loss	(4,300,000)		(20,200,000)
Available-for-sale Debt Securities, Fair Value	1,439,200,000		1,361,000,000
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Less Than 12 Months, Fair Value	50,100,000		222,900,000
Less Than 12 Months, Unrealized Losses	(100,000)		(20,100,000)
12 Months or Longer, Fair Value	154,800,000		2,000,000
12 Months or Longer, Unrealized Losses	(4,200,000)		(100,000)
Total, Fair Value	204,900,000		224,900,000
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Losses Accumulated in Investments	(4,300,000)		(20,200,000)
Bonds and Notes [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Debt Securities, Amortized Cost Basis	2,589,400,000		2,636,400,000
Available-for-sale Debt Securities Gross Unrealized Gain	268,000,000		205,000,000
Available-for-sale Debt Securities, Gross Unrealized Loss	(18,400,000)		(47,700,000)
Available-for-sale Debt Securities, Fair Value	2,839,000,000		2,793,700,000
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Less Than 12 Months, Fair Value	250,800,000		699,800,000
Less Than 12 Months, Unrealized Losses	(2,800,000)		(39,400,000)
12 Months or Longer, Fair Value	409,400,000		103,200,000
12 Months or Longer, Unrealized Losses	(15,600,000)		(8,300,000)
Total, Fair Value	660,200,000		803,000,000
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Losses Accumulated in Investments	(18,400,000)		(47,700,000)
Redeemable Preferred Stocks [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Debt Securities, Amortized Cost Basis	6,200,000		6,600,000
Available-for-sale Debt Securities Gross Unrealized Gain	1,200,000		800,000
Available-for-sale Debt Securities, Gross Unrealized Loss	0		0
Available-for-sale Debt Securities, Fair Value	7,400,000		7,400,000
Collateralized Loan Obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Debt Securities, Amortized Cost Basis	62,500,000		44,200,000
Available-for-sale Debt Securities Gross Unrealized Gain	400,000		500,000
Available-for-sale Debt Securities, Gross Unrealized Loss	(400,000)		0
Available-for-sale Debt Securities, Fair Value	62,500,000		44,700,000
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Less Than 12 Months, Fair Value	36,200,000
Less Than 12 Months, Unrealized Losses	(300,000)
12 Months or Longer, Fair Value	2,200,000
12 Months or Longer, Unrealized Losses	(100,000)
Total, Fair Value	38,400,000
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Losses Accumulated in Investments	(400,000)
Mortgage and Asset-backed [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Debt Securities, Amortized Cost Basis	4,000,000		4,800,000
Available-for-sale Debt Securities Gross Unrealized Gain	1,600,000		1,300,000
Available-for-sale Debt Securities, Gross Unrealized Loss	0		(100,000)
Available-for-sale Debt Securities, Fair Value	5,600,000		6,000,000
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Less Than 12 Months, Fair Value	0		13,900,000
Less Than 12 Months, Unrealized Losses	0		(100,000)
12 Months or Longer, Fair Value	700,000		1,100,000
12 Months or Longer, Unrealized Losses	0		0
Total, Fair Value	700,000		15,000,000
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Losses Accumulated in Investments	0		(100,000)
Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available for sale Securities, Equity Securities, Cost	547,300,000		530,000,000
Available-for-sale Equity Securities, Gross Unrealized Gain	119,900,000		74,700,000
Available-for-sale Equity Securities, Gross Unrealized Loss	(5,800,000)		(6,200,000)
Available for sale Securities, Equity Securities, Fair Value	661,400,000		598,500,000
Available-for-sale Securities, Debt Maturities, Fair Value [Abstract]
Other Receivables	53,400,000		0
Other Liabilities	51,300,000		0
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Less Than 12 Months, Fair Value	174,500,000		166,700,000
Less Than 12 Months, Unrealized Losses	(4,100,000)		(5,500,000)
12 Months or Longer, Fair Value	30,000,000		8,900,000
12 Months or Longer, Unrealized Losses	(1,700,000)		(700,000)
Total, Fair Value	204,500,000		175,600,000
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Losses Accumulated in Investments	(5,800,000)		(6,200,000)
Exchange Traded Funds [Member]
Schedule of Available-for-sale Securities [Line Items]
Available for sale Securities, Equity Securities, Cost	125,200,000		122,000,000
Available-for-sale Equity Securities, Gross Unrealized Gain	5,200,000		3,900,000
Available-for-sale Equity Securities, Gross Unrealized Loss	(600,000)		(1,000,000)
Available for sale Securities, Equity Securities, Fair Value	129,800,000		124,900,000
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Less Than 12 Months, Fair Value	51,200,000		67,600,000
Less Than 12 Months, Unrealized Losses	(200,000)		(1,000,000)
12 Months or Longer, Fair Value	14,500,000		0
12 Months or Longer, Unrealized Losses	(400,000)		0
Total, Fair Value	65,700,000		67,600,000
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Losses Accumulated in Investments	(600,000)		(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	164,300,000		150,300,000
Available-for-sale Equity Securities, Gross Unrealized Gain	55,700,000		25,200,000
Available-for-sale Equity Securities, Gross Unrealized Loss	(2,300,000)		(1,600,000)
Available for sale Securities, Equity Securities, Fair Value	217,700,000		173,900,000
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Less Than 12 Months, Fair Value	74,800,000		53,100,000
Less Than 12 Months, Unrealized Losses	(2,000,000)		(900,000)
12 Months or Longer, Fair Value	2,400,000		5,000,000
12 Months or Longer, Unrealized Losses	(300,000)		(700,000)
Total, Fair Value	77,200,000		58,100,000
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Losses Accumulated in Investments	(2,300,000)		(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	(24,700,000)		(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	(3,800,000)		(5,500,000)
Percentage of Unrealized Loss Postion to Amortized Cost Basis of Available for Sale Security Average	4.00%		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	85,500,000		85,400,000
Available-for-sale Equity Securities, Gross Unrealized Gain	4,900,000		2,900,000
Available-for-sale Equity Securities, Gross Unrealized Loss	(1,000,000)		(2,500,000)
Available for sale Securities, Equity Securities, Fair Value	89,400,000		85,800,000
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Less Than 12 Months, Fair Value	5,000,000		22,500,000
Less Than 12 Months, Unrealized Losses	0		(2,500,000)
12 Months or Longer, Fair Value	12,100,000		2,500,000
12 Months or Longer, Unrealized Losses	(1,000,000)		0
Total, Fair Value	17,100,000		25,000,000
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Losses Accumulated in Investments	(1,000,000)		(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	20,200,000		20,100,000
Available-for-sale Equity Securities, Gross Unrealized Gain	5,600,000		4,400,000
Available-for-sale Equity Securities, Gross Unrealized Loss	(100,000)		(100,000)
Available for sale Securities, Equity Securities, Fair Value	25,700,000		24,400,000
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Less Than 12 Months, Fair Value	700,000		4,300,000
Less Than 12 Months, Unrealized Losses	(100,000)		(100,000)
12 Months or Longer, Fair Value	700,000		700,000
12 Months or Longer, Unrealized Losses	0		0
Total, Fair Value	1,400,000		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	84,800,000		83,400,000
Available-for-sale Equity Securities, Gross Unrealized Gain	25,900,000		21,300,000
Available-for-sale Equity Securities, Gross Unrealized Loss	(1,100,000)		(100,000)
Available for sale Securities, Equity Securities, Fair Value	109,600,000		104,600,000
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Less Than 12 Months, Fair Value	30,800,000		5,000,000
Less Than 12 Months, Unrealized Losses	(1,100,000)		(100,000)
12 Months or Longer, Fair Value	300,000		200,000
12 Months or Longer, Unrealized Losses	0		0
Total, Fair Value	31,100,000		5,200,000
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Losses Accumulated in Investments	(1,100,000)		(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	67,300,000		68,800,000
Available-for-sale Equity Securities, Gross Unrealized Gain	22,600,000		17,000,000
Available-for-sale Equity Securities, Gross Unrealized Loss	(700,000)		(900,000)
Available for sale Securities, Equity Securities, Fair Value	89,200,000		84,900,000
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Less Than 12 Months, Fair Value	12,000,000		14,200,000
Less Than 12 Months, Unrealized Losses	(700,000)		(900,000)
12 Months or Longer, Fair Value	0		500,000
12 Months or Longer, Unrealized Losses	0		0
Total, Fair Value	12,000,000		14,700,000
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Losses Accumulated in Investments	$ (700,000)		$ (900,000)