Investments - Aging of Unrealized Losses on Investments in Fixed Maturities and Equity Securities (Details) - USD ($)	Dec. 31, 2015	Dec. 31, 2014
Schedule of Available-for-sale Securities [Line Items]
Unsettled purchases or sales of Investments in fixed maturities	$ 472,400,000	$ 409,600,000
Other receivables	193,200,000	187,000,000
Fair Value
Less Than 12 Months	1,195,800,000	423,600,000
12 Months or Longer	172,000,000	580,700,000
Total	1,367,800,000	1,004,300,000
Unrealized Losses
Less Than 12 Months	(39,400,000)	(9,700,000)
12 Months or Longer	(17,700,000)	(15,000,000)
Total	(57,100,000)	(24,700,000)
Unrealized losses related to securities for which the Company has recognized credit losses in earnings	200,000	0
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	56,600,000	24,900,000
12 Months or Longer	24,100,000	55,500,000
Total	80,700,000	80,400,000
Unrealized Losses
Less Than 12 Months	(1,600,000)	(700,000)
12 Months or Longer	(2,000,000)	(1,300,000)
Total	(3,600,000)	(2,000,000)
States and Political Subdivisions [Member]
Fair Value
Less Than 12 Months	131,000,000	1,000,000
12 Months or Longer	900,000	126,300,000
Total	131,900,000	127,300,000
Unrealized Losses
Less Than 12 Months	(2,600,000)	0
12 Months or Longer	(100,000)	(1,800,000)
Total	(2,700,000)	(1,800,000)
Corporate Bonds and Notes [Member]
Fair Value
Less Than 12 Months	783,800,000	250,400,000
12 Months or Longer	133,600,000	360,500,000
Total	917,400,000	610,900,000
Unrealized Losses
Less Than 12 Months	(26,000,000)	(5,100,000)
12 Months or Longer	(14,700,000)	(10,000,000)
Total	(40,700,000)	(15,100,000)
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	57,400,000	51,200,000
12 Months or Longer	800,000	3,400,000
Total	58,200,000	54,600,000
Unrealized Losses
Less Than 12 Months	(2,900,000)	(700,000)
12 Months or Longer	(100,000)	(100,000)
Total	(3,000,000)	(800,000)
Other Mortgage and Asset-backed Securities [Member]
Fair Value
Less Than 12 Months	0	0
12 Months or Longer	300,000	400,000
Total	300,000	400,000
Unrealized Losses
Less Than 12 Months	0	0
12 Months or Longer	0	0
Total	0	0
Investments in Fixed Maturities [Member]
Schedule of Available-for-sale Securities [Line Items]
Unsettled purchases or sales of Investments in fixed maturities	5,600,000	0
Other receivables		0
Fair Value
Less Than 12 Months	1,028,800,000	327,500,000
12 Months or Longer	159,700,000	546,100,000
Total	1,188,500,000	873,600,000
Unrealized Losses
Less Than 12 Months	(33,100,000)	(6,500,000)
12 Months or Longer	(16,900,000)	(13,200,000)
Total	(50,000,000)	(19,700,000)
Investments in Fixed Maturities [Member] | External Credit Rating, Investment Grade [Member]
Unrealized Losses
Total	(33,500,000)	(14,100,000)
Investments in Fixed Maturities [Member] | External Credit Rating, Non Investment Grade [Member]
Unrealized Losses
Total	$ (16,500,000)	$ (5,600,000)
Percentage of unrealized loss position to amortized cost basis of available for sale security average	8.10%	4.00%
Preferred Stock [Member] | Finance, Insurance and Real Estate [Member]
Fair Value
Less Than 12 Months	$ 2,700,000	$ 7,500,000
12 Months or Longer	12,300,000	12,100,000
Total	15,000,000	19,600,000
Unrealized Losses
Less Than 12 Months	0	(100,000)
12 Months or Longer	(800,000)	(900,000)
Total	(800,000)	(1,000,000)
Preferred Stock [Member] | All Industries Other than Finance, Insurance and Real Estate [Member]
Fair Value
Less Than 12 Months	7,300,000	0
12 Months or Longer	0	500,000
Total	7,300,000	500,000
Unrealized Losses
Less Than 12 Months	(800,000)	0
12 Months or Longer	0	0
Total	(800,000)	0
Common Stock [Member] | Finance, Insurance and Real Estate [Member]
Fair Value
Less Than 12 Months	16,300,000
12 Months or Longer	0
Total	16,300,000
Unrealized Losses
Less Than 12 Months	(1,000,000)
12 Months or Longer	0
Total	(1,000,000)
Common Stock [Member] | All Industries Other than Finance, Insurance and Real Estate [Member]
Fair Value
Less Than 12 Months	2,800,000
12 Months or Longer	0
Total	2,800,000
Unrealized Losses
Less Than 12 Months	(300,000)
12 Months or Longer	0
Total	(300,000)
Common Stock [Member] | Manufacturing [Member]
Fair Value
Less Than 12 Months		15,100,000
12 Months or Longer		0
Total		15,100,000
Unrealized Losses
Less Than 12 Months		(1,100,000)
12 Months or Longer		0
Total		(1,100,000)
Common Stock [Member] | All Industries Other than Manufacturing [Member]
Fair Value
Less Than 12 Months		4,200,000
12 Months or Longer		1,000,000
Total		5,200,000
Unrealized Losses
Less Than 12 Months		(400,000)
12 Months or Longer		0
Total		(400,000)
Exchange Traded Funds [Member]
Fair Value
Less Than 12 Months	135,200,000	14,900,000
12 Months or Longer	0	14,400,000
Total	135,200,000	29,300,000
Unrealized Losses
Less Than 12 Months	(3,700,000)	(100,000)
12 Months or Longer	0	(600,000)
Total	(3,700,000)	(700,000)
Limited Liability Companies and Limited Partnerships [Member]
Fair Value
Less Than 12 Months	2,700,000	54,400,000
12 Months or Longer	0	6,600,000
Total	2,700,000	61,000,000
Unrealized Losses
Less Than 12 Months	(500,000)	(1,500,000)
12 Months or Longer	0	(300,000)
Total	(500,000)	(1,800,000)
Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Unsettled purchases or sales of Investments in fixed maturities	0	0
Other receivables	0	0
Fair Value
Less Than 12 Months	167,000,000	96,100,000
12 Months or Longer	12,300,000	34,600,000
Total	179,300,000	130,700,000
Unrealized Losses
Less Than 12 Months	(6,300,000)	(3,200,000)
12 Months or Longer	(800,000)	(1,800,000)
Total	$ (7,100,000)	$ (5,000,000)