Investments - Aging of Unrealized Losses on Investments in Fixed Maturities and Equity Securities (Details) - USD ($)	Dec. 31, 2016	Dec. 31, 2015
Schedule of Available-for-sale Securities [Line Items]
Unsettled purchases or sales of Investments in fixed maturities	$ 453,200,000	$ 472,400,000
Other receivables	198,600,000	193,200,000
Fair Value
Less Than 12 Months	1,273,900,000	1,195,800,000
12 Months or Longer	190,300,000	172,000,000
Total	1,464,200,000	1,367,800,000
Unrealized Losses
Less Than 12 Months	(40,500,000)	(39,300,000)
12 Months or Longer	(11,600,000)	(17,700,000)
Total	(52,100,000)	(57,000,000)
Unrealized losses related to securities for which the Company has recognized credit losses in earnings	100,000	200,000
Available-for-sale-Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Securities that have had Recognized Credit Losses in Earnings, Aggregate Losses	0	0
US Government and Government Agencies and Authorities [Member]
Fair Value
Less Than 12 Months	117,700,000	56,600,000
12 Months or Longer	1,100,000	24,100,000
Total	118,800,000	80,700,000
Unrealized Losses
Less Than 12 Months	(7,200,000)	(1,600,000)
12 Months or Longer	0	(2,000,000)
Total	(7,200,000)	(3,600,000)
States and Political Subdivisions [Member]
Fair Value
Less Than 12 Months	432,700,000	131,000,000
12 Months or Longer	300,000	900,000
Total	433,000,000	131,900,000
Unrealized Losses
Less Than 12 Months	(14,100,000)	(2,600,000)
12 Months or Longer	0	(100,000)
Total	(14,100,000)	(2,700,000)
Foreign Government Debt Securities [Member]
Fair Value
Less Than 12 Months	2,100,000
12 Months or Longer	0
Total	2,100,000
Unrealized Losses
Less Than 12 Months	(100,000)
12 Months or Longer	0
Total	(100,000)
Corporate Bonds and Notes [Member]
Fair Value
Less Than 12 Months	663,300,000	783,800,000
12 Months or Longer	107,300,000	133,600,000
Total	770,600,000	917,400,000
Unrealized Losses
Less Than 12 Months	(16,600,000)	(26,000,000)
12 Months or Longer	(7,400,000)	(14,700,000)
Total	(24,000,000)	(40,700,000)
Redeemable Preferred Stocks [Member]
Fair Value
Less Than 12 Months		0
12 Months or Longer		0
Total		0
Unrealized Losses
Less Than 12 Months		0
12 Months or Longer		0
Total		0
Collateralized Loan Obligations [Member]
Fair Value
Less Than 12 Months	19,900,000	57,400,000
12 Months or Longer	21,400,000	800,000
Total	41,300,000	58,200,000
Unrealized Losses
Less Than 12 Months	(700,000)	(2,900,000)
12 Months or Longer	(400,000)	(100,000)
Total	(1,100,000)	(3,000,000)
Other Mortgage and Asset-backed Securities [Member]
Fair Value
Less Than 12 Months		0
12 Months or Longer		300,000
Total		300,000
Unrealized Losses
Less Than 12 Months		0
12 Months or Longer		0
Total		0
Investments in Fixed Maturities [Member]
Schedule of Available-for-sale Securities [Line Items]
Unsettled purchases or sales of Investments in fixed maturities	100,000	5,600,000
Other receivables	2,700,000	0
Fair Value
Less Than 12 Months	1,235,700,000	1,028,800,000
12 Months or Longer	130,100,000	159,700,000
Total	1,365,800,000	1,188,500,000
Unrealized Losses
Less Than 12 Months	(38,700,000)	(33,100,000)
12 Months or Longer	(7,800,000)	(16,900,000)
Total	(46,500,000)	(50,000,000)
Investments in Fixed Maturities [Member] | External Credit Rating, Investment Grade [Member]
Unrealized Losses
Total	(33,800,000)	(33,500,000)
Investments in Fixed Maturities [Member] | External Credit Rating, Non Investment Grade [Member]
Unrealized Losses
Total	$ (12,700,000)	$ (16,500,000)
Percentage of unrealized loss position to amortized cost basis of available for sale security average	5.00%	8.00%
Preferred Stock [Member] | Finance, Insurance and Real Estate [Member]
Fair Value
Less Than 12 Months	$ 15,600,000	$ 2,700,000
12 Months or Longer	7,300,000	12,300,000
Total	22,900,000	15,000,000
Unrealized Losses
Less Than 12 Months	(500,000)	0
12 Months or Longer	(300,000)	(800,000)
Total	(800,000)	(800,000)
Preferred Stock [Member] | All Industries Other than Finance, Insurance and Real Estate [Member]
Fair Value
Less Than 12 Months	5,300,000	7,300,000
12 Months or Longer	0	0
Total	5,300,000	7,300,000
Unrealized Losses
Less Than 12 Months	(500,000)	(800,000)
12 Months or Longer	0	0
Total	(500,000)	(800,000)
Common Stock [Member] | Finance, Insurance and Real Estate [Member]
Fair Value
Less Than 12 Months	2,800,000	16,300,000
12 Months or Longer	0	0
Total	2,800,000	16,300,000
Unrealized Losses
Less Than 12 Months	0	(1,000,000)
12 Months or Longer	0	0
Total	0	(1,000,000)
Common Stock [Member] | All Industries Other than Finance, Insurance and Real Estate [Member]
Fair Value
Less Than 12 Months	600,000	2,800,000
12 Months or Longer	500,000	0
Total	1,100,000	2,800,000
Unrealized Losses
Less Than 12 Months	(100,000)	(200,000)
12 Months or Longer	0	0
Total	(100,000)	(200,000)
Exchange Traded Funds [Member]
Fair Value
Less Than 12 Months	0	135,200,000
12 Months or Longer	18,600,000	0
Total	18,600,000	135,200,000
Unrealized Losses
Less Than 12 Months	0	(3,700,000)
12 Months or Longer	(1,300,000)	0
Total	(1,300,000)	(3,700,000)
Limited Liability Companies and Limited Partnerships [Member]
Fair Value
Less Than 12 Months	13,900,000	2,700,000
12 Months or Longer	33,800,000	0
Total	47,700,000	2,700,000
Unrealized Losses
Less Than 12 Months	(700,000)	(500,000)
12 Months or Longer	(2,200,000)	0
Total	(2,900,000)	(500,000)
Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Unsettled purchases or sales of Investments in fixed maturities	0	0
Other receivables	200,000	0
Fair Value
Less Than 12 Months	38,200,000	167,000,000
12 Months or Longer	60,200,000	12,300,000
Total	98,400,000	179,300,000
Unrealized Losses
Less Than 12 Months	(1,800,000)	(6,200,000)
12 Months or Longer	(3,800,000)	(800,000)
Total	$ (5,600,000)	$ (7,000,000)