Investments - Schedule of Investments (Details) - USD ($)	Jun. 30, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Debt Securities, Amortized Cost Basis	$ 4,906,800,000	$ 4,846,800,000
Available-for-sale Securities, Debt Securities	5,263,900,000	5,124,900,000
Available for sale Securities, Equity Securities, Cost	463,800,000	434,400,000
Available for sale Securities, Equity Securities, Fair Value	500,000,000	481,700,000
Available-for-sale Securities, Debt Maturities, Fair Value [Abstract]
Due in One Year or Less - Amortized Cost	179,400,000
Due in One Year or Less - Fair Value	181,800,000
Due after One Year to Five Years - Amortized Cost	790,200,000
Due after One Year to Five Years - Fair Value	818,000,000
Due after Five Years to Ten Years - Amortized Cost	1,626,900,000
Due after Five Years to Ten Years - Fair Value	1,699,700,000
Due after Ten Years - Amortized Cost	2,028,600,000
Due after Ten Years - Fair Value	2,272,200,000
Asset-backed Securities Not Due in a Single Maturity Date - Amortized Cost	281,700,000
Asset-backed Securities Not Due in a Single Maturity Date - Fair Value	292,200,000
Available-for-sale Securities, Debt Securities	5,263,900,000	5,124,900,000
Other Receivables	190,100,000	198,600,000
Other Liabilities	461,700,000	453,200,000
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	798,800,000	1,273,900,000
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(19,600,000)	(40,500,000)
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	134,700,000	190,300,000
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(7,800,000)	(11,600,000)
Total, Fair Value	933,500,000	1,464,200,000
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Losses Accumulated in Investments	(27,400,000)	(52,100,000)
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	100,000
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	(200,000)	0
US Government and Government Agencies and Authorities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Debt Securities, Amortized Cost Basis	339,900,000	321,200,000
Available-for-sale Debt Securities Gross Unrealized Gain	22,100,000	22,300,000
Available-for-sale Debt Securities, Gross Unrealized Loss	(5,300,000)	(7,200,000)
Available-for-sale Securities, Debt Securities	356,700,000	336,300,000
Available-for-sale Securities, Debt Maturities, Fair Value [Abstract]
Available-for-sale Securities, Debt Securities	356,700,000	336,300,000
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	116,200,000	117,700,000
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(5,300,000)	(7,200,000)
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	1,000,000	1,100,000
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	0	0
Total, Fair Value	117,200,000	118,800,000
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Losses Accumulated in Investments	(5,300,000)	(7,200,000)
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	149,200,000
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	11,900,000
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	4,300,000
Foreign Government Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Debt Securities, Amortized Cost Basis	3,700,000	3,500,000
Available-for-sale Debt Securities Gross Unrealized Gain	100,000	0
Available-for-sale Debt Securities, Gross Unrealized Loss	0	(100,000)
Available-for-sale Securities, Debt Securities	3,800,000	3,400,000
Available-for-sale Securities, Debt Maturities, Fair Value [Abstract]
Available-for-sale Securities, Debt Securities	3,800,000	3,400,000
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	1,200,000	2,100,000
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	0	(100,000)
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	1,200,000	2,100,000
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Losses Accumulated in Investments	0	(100,000)
States and Political Subdivisions [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Debt Securities, Amortized Cost Basis	1,646,000,000	1,640,600,000
Available-for-sale Debt Securities Gross Unrealized Gain	111,000,000	88,400,000
Available-for-sale Debt Securities, Gross Unrealized Loss	(4,400,000)	(14,100,000)
Available-for-sale Securities, Debt Securities	1,752,600,000	1,714,900,000
Available-for-sale Securities, Debt Maturities, Fair Value [Abstract]
Available-for-sale Securities, Debt Securities	1,752,600,000	1,714,900,000
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	217,800,000	432,700,000
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(4,400,000)	(14,100,000)
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	400,000	300,000
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	0	0
Total, Fair Value	218,200,000	433,000,000
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Losses Accumulated in Investments	(4,400,000)	(14,100,000)
Corporate Bonds and Notes [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Debt Securities, Amortized Cost Basis	2,796,200,000	2,758,900,000
Available-for-sale Debt Securities Gross Unrealized Gain	241,700,000	209,900,000
Available-for-sale Debt Securities, Gross Unrealized Loss	(13,000,000)	(24,000,000)
Available-for-sale Securities, Debt Securities	3,024,900,000	2,944,800,000
Available-for-sale Securities, Debt Maturities, Fair Value [Abstract]
Available-for-sale Securities, Debt Securities	3,024,900,000	2,944,800,000
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	355,700,000	663,300,000
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(7,700,000)	(16,600,000)
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	93,700,000	107,300,000
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(5,300,000)	(7,400,000)
Total, Fair Value	449,400,000	770,600,000
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Losses Accumulated in Investments	(13,000,000)	(24,000,000)
Redeemable Preferred Stocks [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Debt Securities, Amortized Cost Basis	100,000	500,000
Available-for-sale Debt Securities Gross Unrealized Gain	0	100,000
Available-for-sale Debt Securities, Gross Unrealized Loss	0	0
Available-for-sale Securities, Debt Securities	100,000	600,000
Available-for-sale Securities, Debt Maturities, Fair Value [Abstract]
Available-for-sale Securities, Debt Securities	100,000	600,000
Collateralized Loan Obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Debt Securities, Amortized Cost Basis	120,500,000	121,200,000
Available-for-sale Debt Securities Gross Unrealized Gain	4,200,000	2,700,000
Available-for-sale Debt Securities, Gross Unrealized Loss	0	(1,100,000)
Available-for-sale Securities, Debt Securities	124,700,000	122,800,000
Available-for-sale Securities, Debt Maturities, Fair Value [Abstract]
Available-for-sale Securities, Debt Securities	124,700,000	122,800,000
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	1,500,000	19,900,000
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	0	(700,000)
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	5,100,000	21,400,000
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	0	(400,000)
Total, Fair Value	6,600,000	41,300,000
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Losses Accumulated in Investments	0	(1,100,000)
Other Mortgage- and Asset-backed [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Debt Securities, Amortized Cost Basis	400,000	900,000
Available-for-sale Debt Securities Gross Unrealized Gain	700,000	1,200,000
Available-for-sale Debt Securities, Gross Unrealized Loss	0	0
Available-for-sale Securities, Debt Securities	1,100,000	2,100,000
Available-for-sale Securities, Debt Maturities, Fair Value [Abstract]
Available-for-sale Securities, Debt Securities	1,100,000	2,100,000
Fixed Maturities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Debt Securities, Amortized Cost Basis	4,906,800,000	4,846,800,000
Available-for-sale Debt Securities Gross Unrealized Gain	379,800,000	324,600,000
Available-for-sale Debt Securities, Gross Unrealized Loss	(22,700,000)	(46,500,000)
Available-for-sale Securities, Debt Securities	5,263,900,000	5,124,900,000
Available-for-sale Securities, Debt Maturities, Fair Value [Abstract]
Available-for-sale Securities, Debt Securities	5,263,900,000	5,124,900,000
Other Receivables	0	2,700,000
Other Liabilities	6,300,000	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	692,400,000	1,235,700,000
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(17,400,000)	(38,700,000)
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	100,200,000	130,100,000
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(5,300,000)	(7,800,000)
Total, Fair Value	792,600,000	1,365,800,000
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Losses Accumulated in Investments	(22,700,000)	(46,500,000)
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	(14,000,000)	(33,800,000)
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	$ (8,700,000)	$ (12,700,000)
Percentage of Unrealized Loss Position to Amortized Cost Basis of Available for Sale Security Average	5.00%	5.00%
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	$ 126,800,000
Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available for sale Securities, Equity Securities, Cost	463,800,000	$ 434,400,000
Available-for-sale Equity Securities, Gross Unrealized Gain	40,900,000	52,900,000
Available-for-sale Equity Securities, Gross Unrealized Loss	(4,700,000)	(5,600,000)
Available for sale Securities, Equity Securities, Fair Value	500,000,000	481,700,000
Available-for-sale Securities, Debt Maturities, Fair Value [Abstract]
Other Receivables	3,900,000	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	106,400,000	38,200,000
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(2,200,000)	(1,800,000)
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	34,500,000	60,200,000
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(2,500,000)	(3,800,000)
Total, Fair Value	140,900,000	98,400,000
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Losses Accumulated in Investments	(4,700,000)	(5,600,000)
Preferred Stock [Member] | Finance, Insurance and Real Estate [Member]
Schedule of Available-for-sale Securities [Line Items]
Available for sale Securities, Equity Securities, Cost	53,600,000	58,100,000
Available-for-sale Equity Securities, Gross Unrealized Gain	4,900,000	2,300,000
Available-for-sale Equity Securities, Gross Unrealized Loss	0	(800,000)
Available for sale Securities, Equity Securities, Fair Value	58,500,000	59,600,000
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	0	15,600,000
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	0	(500,000)
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	200,000	7,300,000
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	0	(300,000)
Total, Fair Value	200,000	22,900,000
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Losses Accumulated in Investments	0	(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	18,300,000	18,500,000
Available-for-sale Equity Securities, Gross Unrealized Gain	5,000,000	4,900,000
Available-for-sale Equity Securities, Gross Unrealized Loss	0	(500,000)
Available for sale Securities, Equity Securities, Fair Value	23,300,000	22,900,000
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	200,000	5,300,000
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	0	(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	200,000	5,300,000
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Losses Accumulated in Investments	0	(500,000)
Common Stocks [Member] | Finance, Insurance and Real Estate [Member]
Schedule of Available-for-sale Securities [Line Items]
Available for sale Securities, Equity Securities, Cost	6,700,000	31,200,000
Available-for-sale Equity Securities, Gross Unrealized Gain	1,000,000	2,300,000
Available-for-sale Equity Securities, Gross Unrealized Loss	0	0
Available for sale Securities, Equity Securities, Fair Value	7,700,000	33,500,000
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value		2,800,000
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		2,800,000
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Losses Accumulated in Investments		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	9,100,000	7,200,000
Available-for-sale Equity Securities, Gross Unrealized Gain	3,500,000	4,600,000
Available-for-sale Equity Securities, Gross Unrealized Loss	(300,000)	(100,000)
Available for sale Securities, Equity Securities, Fair Value	12,300,000	11,700,000
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	800,000	600,000
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(300,000)	(100,000)
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	1,300,000	1,100,000
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Losses Accumulated in Investments	(300,000)	(100,000)
Exchange Traded Funds [Member]
Schedule of Available-for-sale Securities [Line Items]
Available for sale Securities, Equity Securities, Cost	193,900,000	136,100,000
Available-for-sale Equity Securities, Gross Unrealized Gain	4,200,000	9,600,000
Available-for-sale Equity Securities, Gross Unrealized Loss	(600,000)	(1,300,000)
Available for sale Securities, Equity Securities, Fair Value	197,500,000	144,400,000
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	73,700,000	0
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(600,000)	0
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	0	18,600,000
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	0	(1,300,000)
Total, Fair Value	73,700,000	18,600,000
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Losses Accumulated in Investments	(600,000)	(1,300,000)
Limited Liability Companies and Limited Partnerships [Member]
Schedule of Available-for-sale Securities [Line Items]
Available for sale Securities, Equity Securities, Cost	182,200,000	183,300,000
Available-for-sale Equity Securities, Gross Unrealized Gain	22,300,000	29,200,000
Available-for-sale Equity Securities, Gross Unrealized Loss	(3,800,000)	(2,900,000)
Available for sale Securities, Equity Securities, Fair Value	200,700,000	209,600,000
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	31,700,000	13,900,000
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(1,300,000)	(700,000)
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	33,800,000	33,800,000
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(2,500,000)	(2,200,000)
Total, Fair Value	65,500,000	47,700,000
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Losses Accumulated in Investments	$ (3,800,000)	$ (2,900,000)