Investments - Schedule of Investments (Details) (USD $)	6 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Dec. 31, 2013
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Debt Securities, Amortized Cost Basis	$ 4,294,800,000
Available-for-sale Debt Securities, Fair Value	4,680,000,000		4,575,000,000
Available for sale Securities, Equity Securities, Cost	541,000,000		530,000,000
Available for sale Securities, Equity Securities, Fair Value	646,200,000		598,500,000
Available-for-sale Securities, Debt Maturities, Fair Value [Abstract]
Due in One Year or Less - Amortized Cost	48,200,000
Due in One Year or Less - Fair Value	49,100,000
Due after One Year to Five Years - Amortized Cost	770,500,000
Due after One Year to Five Years - Fair Value	824,500,000
Due after Five Years to Ten Years - Amortized Cost	1,290,900,000
Due after Five Years to Ten Years - Fair Value	1,349,100,000
Due after Ten Years - Amortized Cost	1,986,600,000
Due after Ten Years - Fair Value	2,246,100,000
Asset-backed Securities Not Due in a Single Maturity Date - Amortized Cost	198,600,000
Asset-backed Securities Not Due in a Single Maturity Date - Fair Value	211,200,000
Other Receivables	190,500,000		193,100,000
Other Liabilities	390,100,000		329,800,000
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Less Than 12 Months, Fair Value	251,100,000		1,199,100,000
Less Than 12 Months, Unrealized Losses	(5,700,000)		(76,000,000)
12 Months or Longer, Fair Value	780,900,000		119,600,000
12 Months or Longer, Unrealized Losses	(25,800,000)		(9,900,000)
Total, Fair Value	1,032,000,000		1,318,700,000
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Losses Accumulated in Investments	(31,500,000)		(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	42,900,000	0
Sales of Fair Value Option Investments	2,900,000	0
Fair Value Option Investments	40,300,000		0
Fixed Maturities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Debt Securities, Amortized Cost Basis	4,294,800,000		4,370,500,000
Available-for-sale Debt Securities Gross Unrealized Gain	412,600,000		284,200,000
Available-for-sale Debt Securities, Gross Unrealized Loss	(27,400,000)		(79,700,000)
Available-for-sale Debt Securities, Fair Value	4,680,000,000		4,575,000,000
Available-for-sale Securities, Debt Maturities, Fair Value [Abstract]
Other Receivables	0		2,500,000
Other Liabilities	46,300,000		0
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Less Than 12 Months, Fair Value	172,300,000		1,032,400,000
Less Than 12 Months, Unrealized Losses	(2,900,000)		(70,500,000)
12 Months or Longer, Fair Value	750,000,000		110,700,000
12 Months or Longer, Unrealized Losses	(24,500,000)		(9,200,000)
Total, Fair Value	922,300,000		1,143,100,000
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Losses Accumulated in Investments	(27,400,000)		(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	332,400,000		351,100,000
Available-for-sale Debt Securities Gross Unrealized Gain	28,900,000		22,800,000
Available-for-sale Debt Securities, Gross Unrealized Loss	(6,000,000)		(11,700,000)
Available-for-sale Debt Securities, Fair Value	355,300,000		362,200,000
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Less Than 12 Months, Fair Value	24,100,000		95,800,000
Less Than 12 Months, Unrealized Losses	(1,500,000)		(10,900,000)
12 Months or Longer, Fair Value	65,400,000		4,400,000
12 Months or Longer, Unrealized Losses	(4,500,000)		(800,000)
Total, Fair Value	89,500,000		100,200,000
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Losses Accumulated in Investments	(6,000,000)		(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	145,800,000
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	10,100,000
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	55,000,000
States and Political Subdivisions [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Debt Securities, Amortized Cost Basis	1,295,300,000		1,327,400,000
Available-for-sale Debt Securities Gross Unrealized Gain	99,800,000		53,800,000
Available-for-sale Debt Securities, Gross Unrealized Loss	(5,900,000)		(20,200,000)
Available-for-sale Debt Securities, Fair Value	1,389,200,000		1,361,000,000
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Less Than 12 Months, Fair Value	31,000,000		222,900,000
Less Than 12 Months, Unrealized Losses	(400,000)		(20,100,000)
12 Months or Longer, Fair Value	181,000,000		2,000,000
12 Months or Longer, Unrealized Losses	(5,500,000)		(100,000)
Total, Fair Value	212,000,000		224,900,000
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Losses Accumulated in Investments	(5,900,000)		(20,200,000)
Bonds and Notes [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Debt Securities, Amortized Cost Basis	2,607,600,000		2,636,400,000
Available-for-sale Debt Securities Gross Unrealized Gain	281,100,000		205,000,000
Available-for-sale Debt Securities, Gross Unrealized Loss	(15,300,000)		(47,700,000)
Available-for-sale Debt Securities, Fair Value	2,873,400,000		2,793,700,000
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Less Than 12 Months, Fair Value	104,100,000		699,800,000
Less Than 12 Months, Unrealized Losses	(900,000)		(39,400,000)
12 Months or Longer, Fair Value	500,600,000		103,200,000
12 Months or Longer, Unrealized Losses	(14,400,000)		(8,300,000)
Total, Fair Value	604,700,000		803,000,000
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Losses Accumulated in Investments	(15,300,000)		(47,700,000)
Redeemable Preferred Stocks [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Debt Securities, Amortized Cost Basis	6,400,000		6,600,000
Available-for-sale Debt Securities Gross Unrealized Gain	700,000		800,000
Available-for-sale Debt Securities, Gross Unrealized Loss	0		0
Available-for-sale Debt Securities, Fair Value	7,100,000		7,400,000
Collateralized Loan Obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Debt Securities, Amortized Cost Basis	49,100,000		44,200,000
Available-for-sale Debt Securities Gross Unrealized Gain	600,000		500,000
Available-for-sale Debt Securities, Gross Unrealized Loss	(200,000)		0
Available-for-sale Debt Securities, Fair Value	49,500,000		44,700,000
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Less Than 12 Months, Fair Value	13,100,000
Less Than 12 Months, Unrealized Losses	(100,000)
12 Months or Longer, Fair Value	2,300,000
12 Months or Longer, Unrealized Losses	(100,000)
Total, Fair Value	15,400,000
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Losses Accumulated in Investments	(200,000)
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	1,500,000		1,300,000
Available-for-sale Debt Securities, Gross Unrealized Loss	0		(100,000)
Available-for-sale Debt Securities, Fair Value	5,500,000		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	541,000,000		530,000,000
Available-for-sale Equity Securities, Gross Unrealized Gain	109,300,000		74,700,000
Available-for-sale Equity Securities, Gross Unrealized Loss	4,100,000		6,200,000
Available for sale Securities, Equity Securities, Fair Value	646,200,000		598,500,000
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Less Than 12 Months, Fair Value	78,800,000		166,700,000
Less Than 12 Months, Unrealized Losses	(2,800,000)		(5,500,000)
12 Months or Longer, Fair Value	30,900,000		8,900,000
12 Months or Longer, Unrealized Losses	(1,300,000)		(700,000)
Total, Fair Value	109,700,000		175,600,000
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Losses Accumulated in Investments	(4,100,000)		(6,200,000)
Exchange Traded Funds [Member]
Schedule of Available-for-sale Securities [Line Items]
Available for sale Securities, Equity Securities, Cost	119,000,000		122,000,000
Available-for-sale Equity Securities, Gross Unrealized Gain	12,000,000		3,900,000
Available-for-sale Equity Securities, Gross Unrealized Loss	100,000		1,000,000
Available for sale Securities, Equity Securities, Fair Value	130,900,000		124,900,000
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Less Than 12 Months, Fair Value	0		67,600,000
Less Than 12 Months, Unrealized Losses	0		(1,000,000)
12 Months or Longer, Fair Value	14,900,000		0
12 Months or Longer, Unrealized Losses	(100,000)		0
Total, Fair Value	14,900,000		67,600,000
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Losses Accumulated in Investments	(100,000)		(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	160,300,000		150,300,000
Available-for-sale Equity Securities, Gross Unrealized Gain	36,300,000		25,200,000
Available-for-sale Equity Securities, Gross Unrealized Loss	2,200,000		1,600,000
Available for sale Securities, Equity Securities, Fair Value	194,400,000		173,900,000
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Less Than 12 Months, Fair Value	50,400,000		53,100,000
Less Than 12 Months, Unrealized Losses	(1,800,000)		(900,000)
12 Months or Longer, Fair Value	2,800,000		5,000,000
12 Months or Longer, Unrealized Losses	(400,000)		(700,000)
Total, Fair Value	53,200,000		58,100,000
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Losses Accumulated in Investments	(2,200,000)		(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	(23,900,000)		(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	(3,500,000)		(5,500,000)
Percentage of Unrealized Loss Postion to Amortized Cost Basis of Available for Sale Security Average	3.00%		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	5,500,000		2,900,000
Available-for-sale Equity Securities, Gross Unrealized Loss	800,000		2,500,000
Available for sale Securities, Equity Securities, Fair Value	90,100,000		85,800,000
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Less Than 12 Months, Fair Value	0		22,500,000
Less Than 12 Months, Unrealized Losses	0		(2,500,000)
12 Months or Longer, Fair Value	12,200,000		2,500,000
12 Months or Longer, Unrealized Losses	(800,000)		0
Total, Fair Value	12,200,000		25,000,000
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Losses Accumulated in Investments	(800,000)		(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	5,500,000		4,400,000
Available-for-sale Equity Securities, Gross Unrealized Loss	100,000		100,000
Available for sale Securities, Equity Securities, Fair Value	26,500,000		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	700,000		700,000
12 Months or Longer, Unrealized Losses	0		0
Total, Fair Value	5,000,000		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	83,400,000		83,400,000
Available-for-sale Equity Securities, Gross Unrealized Gain	26,600,000		21,300,000
Available-for-sale Equity Securities, Gross Unrealized Loss	200,000		100,000
Available for sale Securities, Equity Securities, Fair Value	109,800,000		104,600,000
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Less Than 12 Months, Fair Value	10,800,000		5,000,000
Less Than 12 Months, Unrealized Losses	(200,000)		(100,000)
12 Months or Longer, Fair Value	300,000		200,000
12 Months or Longer, Unrealized Losses	0		0
Total, Fair Value	11,100,000		5,200,000
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Losses Accumulated in Investments	(200,000)		(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	71,800,000		68,800,000
Available-for-sale Equity Securities, Gross Unrealized Gain	23,400,000		17,000,000
Available-for-sale Equity Securities, Gross Unrealized Loss	700,000		900,000
Available for sale Securities, Equity Securities, Fair Value	94,500,000		84,900,000
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Less Than 12 Months, Fair Value	13,300,000		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	13,300,000		14,700,000
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Losses Accumulated in Investments	$ (700,000)		$ (900,000)