Investments - Schedule of Investments (Details) - USD ($)	Jun. 30, 2016	Dec. 31, 2015
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Debt Securities, Amortized Cost Basis	$ 4,579,700,000	$ 4,560,700,000
Fair Value	5,084,300,000	4,852,300,000
Available for sale Securities, Equity Securities, Cost	460,800,000	486,900,000
Available for sale Securities, Equity Securities, Fair Value	509,600,000	523,200,000
Available-for-sale Securities, Debt Maturities, Fair Value [Abstract]
Due in One Year or Less - Amortized Cost	36,000,000
Due in One Year or Less - Fair Value	36,600,000
Due after One Year to Five Years - Amortized Cost	926,100,000
Due after One Year to Five Years - Fair Value	970,500,000
Due after Five Years to Ten Years - Amortized Cost	1,497,400,000
Due after Five Years to Ten Years - Fair Value	1,605,100,000
Due after Ten Years - Amortized Cost	1,898,300,000
Due after Ten Years - Fair Value	2,245,300,000
Asset-backed Securities Not Due in a Single Maturity Date - Amortized Cost	221,900,000
Asset-backed Securities Not Due in a Single Maturity Date - Fair Value	226,800,000
Fair Value	5,084,300,000	4,852,300,000
Other Receivables	192,700,000	193,200,000
Other Liabilities	491,000,000	472,400,000
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	310,300,000	1,195,800,000
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(16,300,000)	(39,300,000)
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	206,400,000	172,000,000
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(12,100,000)	(17,700,000)
Total, Fair Value	516,700,000	1,367,800,000
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Losses Accumulated in Investments	(28,400,000)	(57,000,000)
Available-for-sale-Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Securities that have had Recognized Credit Losses in Earnings, Aggregate Losses	600,000	200,000
Available For Sale Securities, Continuous Unrealized Loss Position, Credit Losses Recognized Related to Securities 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	4,579,700,000	4,560,700,000
Available-for-sale Debt Securities Gross Unrealized Gain	525,200,000	341,600,000
Available-for-sale Debt Securities, Gross Unrealized Loss	(20,600,000)	(50,000,000)
Fair Value	5,084,300,000	4,852,300,000
Available-for-sale Securities, Debt Maturities, Fair Value [Abstract]
Fair Value	5,084,300,000	4,852,300,000
Other Receivables	0
Other Liabilities	1,300,000	5,600,000
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	191,900,000	1,028,800,000
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(9,100,000)	(33,100,000)
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	195,800,000	159,700,000
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(11,500,000)	(16,900,000)
Total, Fair Value	387,700,000	1,188,500,000
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Losses Accumulated in Investments	(20,600,000)	(50,000,000)
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	(4,000,000)	(33,500,000)
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	$ (16,600,000)	$ (16,500,000)
Percentage of Unrealized Loss Position to Amortized Cost Basis of Available for Sale Security Average	7.00%	8.00%
US Government and Government Agencies and Authorities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Debt Securities, Amortized Cost Basis	$ 258,500,000	$ 298,000,000
Available-for-sale Debt Securities Gross Unrealized Gain	35,700,000	26,200,000
Available-for-sale Debt Securities, Gross Unrealized Loss	0	(3,600,000)
Fair Value	294,200,000	320,600,000
Available-for-sale Securities, Debt Maturities, Fair Value [Abstract]
Fair Value	294,200,000	320,600,000
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	0	56,600,000
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	0	(1,600,000)
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	1,200,000	24,100,000
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	0	(2,000,000)
Total, Fair Value	1,200,000	80,700,000
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Losses Accumulated in Investments	0	(3,600,000)
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	98,100,000
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	15,900,000
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	5,700,000
States and Political Subdivisions [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Debt Securities, Amortized Cost Basis	1,470,600,000	1,513,700,000
Available-for-sale Debt Securities Gross Unrealized Gain	176,800,000	111,600,000
Available-for-sale Debt Securities, Gross Unrealized Loss	(500,000)	(2,700,000)
Fair Value	1,646,900,000	1,622,600,000
Available-for-sale Securities, Debt Maturities, Fair Value [Abstract]
Fair Value	1,646,900,000	1,622,600,000
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	6,000,000	131,000,000
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(500,000)	(2,600,000)
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	1,300,000	900,000
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	0	(100,000)
Total, Fair Value	7,300,000	131,900,000
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Losses Accumulated in Investments	(500,000)	(2,700,000)
Bonds and Notes [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Debt Securities, Amortized Cost Basis	2,736,700,000	2,651,500,000
Available-for-sale Debt Securities Gross Unrealized Gain	311,100,000	202,000,000
Available-for-sale Debt Securities, Gross Unrealized Loss	(14,600,000)	(40,700,000)
Fair Value	3,033,200,000	2,812,800,000
Available-for-sale Securities, Debt Maturities, Fair Value [Abstract]
Fair Value	3,033,200,000	2,812,800,000
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	116,400,000	783,800,000
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(4,400,000)	(26,000,000)
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	182,000,000	133,600,000
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(10,200,000)	(14,700,000)
Total, Fair Value	298,400,000	917,400,000
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Losses Accumulated in Investments	(14,600,000)	(40,700,000)
Redeemable Preferred Stocks [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Debt Securities, Amortized Cost Basis	3,100,000	3,700,000
Available-for-sale Debt Securities Gross Unrealized Gain	0	100,000
Available-for-sale Debt Securities, Gross Unrealized Loss	(200,000)	0
Fair Value	2,900,000	3,800,000
Available-for-sale Securities, Debt Maturities, Fair Value [Abstract]
Fair Value	2,900,000	3,800,000
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	2,700,000
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(200,000)
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	2,700,000
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Losses Accumulated in Investments	(200,000)
Collateralized Loan Obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Debt Securities, Amortized Cost Basis	107,100,000	90,000,000
Available-for-sale Debt Securities Gross Unrealized Gain	300,000	300,000
Available-for-sale Debt Securities, Gross Unrealized Loss	(5,200,000)	(3,000,000)
Fair Value	102,200,000	87,300,000
Available-for-sale Securities, Debt Maturities, Fair Value [Abstract]
Fair Value	102,200,000	87,300,000
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	66,800,000	57,400,000
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(3,900,000)	(2,900,000)
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	11,100,000	800,000
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(1,300,000)	(100,000)
Total, Fair Value	77,900,000	58,200,000
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Losses Accumulated in Investments	(5,200,000)	(3,000,000)
Other Mortgage- and Asset-backed [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Debt Securities, Amortized Cost Basis	3,700,000	3,800,000
Available-for-sale Debt Securities Gross Unrealized Gain	1,300,000	1,400,000
Available-for-sale Debt Securities, Gross Unrealized Loss	(100,000)	0
Fair Value	4,900,000	5,200,000
Available-for-sale Securities, Debt Maturities, Fair Value [Abstract]
Fair Value	4,900,000	5,200,000
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	0	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	200,000	300,000
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	0	0
Total, Fair Value	200,000	300,000
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Losses Accumulated in Investments	(100,000)	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	107,100,000
Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available for sale Securities, Equity Securities, Cost	460,800,000	486,900,000
Available-for-sale Equity Securities, Gross Unrealized Gain	56,600,000	43,300,000
Available-for-sale Equity Securities, Gross Unrealized Loss	(7,800,000)	(7,000,000)
Available for sale Securities, Equity Securities, Fair Value	509,600,000	523,200,000
Available-for-sale Securities, Debt Maturities, Fair Value [Abstract]
Other Receivables	200,000
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	118,400,000	167,000,000
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(7,200,000)	(6,200,000)
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	10,600,000	12,300,000
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(600,000)	(800,000)
Total, Fair Value	129,000,000	179,300,000
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Losses Accumulated in Investments	(7,800,000)	(7,000,000)
Preferred Stock [Member] | Finance, Insurance and Real Estate [Member]
Schedule of Available-for-sale Securities [Line Items]
Available for sale Securities, Equity Securities, Cost	76,100,000	80,800,000
Available-for-sale Equity Securities, Gross Unrealized Gain	4,900,000	4,900,000
Available-for-sale Equity Securities, Gross Unrealized Loss	(600,000)	(800,000)
Available for sale Securities, Equity Securities, Fair Value	80,400,000	84,900,000
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	2,000,000	2,700,000
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	10,100,000	12,300,000
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(600,000)	(800,000)
Total, Fair Value	12,100,000	15,000,000
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Losses Accumulated in Investments	(600,000)	(800,000)
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	16,000,000	17,100,000
Available-for-sale Equity Securities, Gross Unrealized Gain	3,800,000	2,700,000
Available-for-sale Equity Securities, Gross Unrealized Loss	(600,000)	(800,000)
Available for sale Securities, Equity Securities, Fair Value	19,200,000	19,000,000
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	7,900,000	7,300,000
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(600,000)	(800,000)
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	7,900,000	7,300,000
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Losses Accumulated in Investments	(600,000)	(800,000)
Common Stocks [Member] | Finance, Insurance and Real Estate [Member]
Schedule of Available-for-sale Securities [Line Items]
Available for sale Securities, Equity Securities, Cost	27,600,000	18,900,000
Available-for-sale Equity Securities, Gross Unrealized Gain	8,100,000	5,300,000
Available-for-sale Equity Securities, Gross Unrealized Loss	(300,000)	(1,000,000)
Available for sale Securities, Equity Securities, Fair Value	35,400,000	23,200,000
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	3,200,000	16,300,000
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(300,000)	(1,000,000)
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	3,200,000	16,300,000
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Losses Accumulated in Investments	(300,000)	(1,000,000)
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	9,200,000	9,400,000
Available-for-sale Equity Securities, Gross Unrealized Gain	5,100,000	4,300,000
Available-for-sale Equity Securities, Gross Unrealized Loss	(100,000)	(200,000)
Available for sale Securities, Equity Securities, Fair Value	14,200,000	13,500,000
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	2,000,000	2,800,000
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(100,000)	(200,000)
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	500,000	0
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	0	0
Total, Fair Value	2,500,000	2,800,000
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Losses Accumulated in Investments	(100,000)	(200,000)
Exchange Traded Funds [Member]
Schedule of Available-for-sale Securities [Line Items]
Available for sale Securities, Equity Securities, Cost	145,100,000	179,700,000
Available-for-sale Equity Securities, Gross Unrealized Gain	2,600,000	1,100,000
Available-for-sale Equity Securities, Gross Unrealized Loss	(2,000,000)	(3,700,000)
Available for sale Securities, Equity Securities, Fair Value	145,700,000	177,100,000
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	32,000,000	135,200,000
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(2,000,000)	(3,700,000)
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	32,000,000	135,200,000
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Losses Accumulated in Investments	(2,000,000)	(3,700,000)
Limited Liability Companies and Limited Partnerships [Member]
Schedule of Available-for-sale Securities [Line Items]
Available for sale Securities, Equity Securities, Cost	186,800,000	181,000,000
Available-for-sale Equity Securities, Gross Unrealized Gain	32,100,000	25,000,000
Available-for-sale Equity Securities, Gross Unrealized Loss	(4,200,000)	(500,000)
Available for sale Securities, Equity Securities, Fair Value	214,700,000	205,500,000
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	71,300,000	2,700,000
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(4,200,000)	(500,000)
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	71,300,000	2,700,000
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Losses Accumulated in Investments	$ (4,200,000)	$ (500,000)