Investments - Aging of Unrealized Losses on Investments in Fixed Maturities and Equity Securities (Details) - USD ($)	Dec. 31, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Unsettled purchases or sales of Investments in fixed maturities	$ 453,700,000	$ 453,200,000
Other receivables	194,300,000	198,600,000
Fair Value
Less Than 12 Months	533,800,000	1,273,900,000
12 Months or Longer	447,500,000	190,300,000
Total	981,300,000	1,464,200,000
Unrealized Losses
Less Than 12 Months	(8,100,000)	(40,500,000)
12 Months or Longer	(17,800,000)	(11,600,000)
Total	(25,900,000)	(52,100,000)
Unrealized losses related to securities for which the Company has recognized credit losses in earnings	0	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	0	0
US Government and Government Agencies and Authorities [Member]
Fair Value
Less Than 12 Months	140,000,000	117,700,000
12 Months or Longer	103,400,000	1,100,000
Total	243,400,000	118,800,000
Unrealized Losses
Less Than 12 Months	(1,100,000)	(7,200,000)
12 Months or Longer	(5,100,000)	0
Total	(6,200,000)	(7,200,000)
States and Political Subdivisions [Member]
Fair Value
Less Than 12 Months	57,000,000	432,700,000
12 Months or Longer	124,200,000	300,000
Total	181,200,000	433,000,000
Unrealized Losses
Less Than 12 Months	(300,000)	(14,100,000)
12 Months or Longer	(2,000,000)	0
Total	(2,300,000)	(14,100,000)
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	283,100,000	663,300,000
12 Months or Longer	208,900,000	107,300,000
Total	492,000,000	770,600,000
Unrealized Losses
Less Than 12 Months	(4,200,000)	(16,600,000)
12 Months or Longer	(6,800,000)	(7,400,000)
Total	(11,000,000)	(24,000,000)
Collateralized Loan Obligations [Member]
Fair Value
Less Than 12 Months	2,800,000	19,900,000
12 Months or Longer	2,400,000	21,400,000
Total	5,200,000	41,300,000
Unrealized Losses
Less Than 12 Months	0	(700,000)
12 Months or Longer	0	(400,000)
Total	0	(1,100,000)
Investments in Fixed Maturities [Member]
Schedule of Available-for-sale Securities [Line Items]
Unsettled purchases or sales of Investments in fixed maturities	5,400,000	100,000
Other receivables	0	2,700,000
Fair Value
Less Than 12 Months	482,900,000	1,235,700,000
12 Months or Longer	438,900,000	130,100,000
Total	921,800,000	1,365,800,000
Unrealized Losses
Less Than 12 Months	(5,600,000)	(38,700,000)
12 Months or Longer	(13,900,000)	(7,800,000)
Total	(19,500,000)	(46,500,000)
Investments in Fixed Maturities [Member] | External Credit Rating, Investment Grade [Member]
Unrealized Losses
Total	12,600,000	(33,800,000)
Investments in Fixed Maturities [Member] | External Credit Rating, Non Investment Grade [Member]
Unrealized Losses
Total	$ 6,900,000	$ (12,700,000)
Percentage of unrealized loss position to amortized cost basis of available for sale security average	4.00%	5.00%
Preferred Stock [Member] | Finance, Insurance and Real Estate [Member]
Fair Value
Less Than 12 Months	$ 300,000	$ 15,600,000
12 Months or Longer	200,000	7,300,000
Total	500,000	22,900,000
Unrealized Losses
Less Than 12 Months	0	(500,000)
12 Months or Longer	0	(300,000)
Total	0	(800,000)
Preferred Stock [Member] | All Industries Other than Finance, Insurance and Real Estate [Member]
Fair Value
Less Than 12 Months	200,000	5,300,000
12 Months or Longer	200,000	0
Total	400,000	5,300,000
Unrealized Losses
Less Than 12 Months	0	(500,000)
12 Months or Longer	0	0
Total	0	(500,000)
Common Stock [Member] | Finance, Insurance and Real Estate [Member]
Fair Value
Less Than 12 Months	1,600,000	2,800,000
12 Months or Longer	0	0
Total	1,600,000	2,800,000
Unrealized Losses
Less Than 12 Months	(100,000)	0
12 Months or Longer	0	0
Total	(100,000)	0
Common Stock [Member] | All Industries Other than Finance, Insurance and Real Estate [Member]
Fair Value
Less Than 12 Months	400,000	600,000
12 Months or Longer	500,000	500,000
Total	900,000	1,100,000
Unrealized Losses
Less Than 12 Months	(300,000)	(100,000)
12 Months or Longer	(100,000)	0
Total	(400,000)	(100,000)
Exchange Traded Funds [Member]
Fair Value
Less Than 12 Months		0
12 Months or Longer		18,600,000
Total		18,600,000
Unrealized Losses
Less Than 12 Months		0
12 Months or Longer		(1,300,000)
Total		(1,300,000)
Limited Liability Companies and Limited Partnerships [Member]
Fair Value
Less Than 12 Months	48,400,000	13,900,000
12 Months or Longer	7,700,000	33,800,000
Total	56,100,000	47,700,000
Unrealized Losses
Less Than 12 Months	(2,100,000)	(700,000)
12 Months or Longer	(3,800,000)	(2,200,000)
Total	(5,900,000)	(2,900,000)
Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Unsettled purchases or sales of Investments in fixed maturities	0
Other receivables	0	200,000
Fair Value
Less Than 12 Months	50,900,000	38,200,000
12 Months or Longer	8,600,000	60,200,000
Total	59,500,000	98,400,000
Unrealized Losses
Less Than 12 Months	(2,500,000)	(1,800,000)
12 Months or Longer	(3,900,000)	(3,800,000)
Total	$ (6,400,000)	$ (5,600,000)