Investments - Aging of Unrealized Losses on Investments in Fixed Maturities and Equity Securities (Details) - USD ($)	Dec. 31, 2018	Dec. 31, 2017
Unrealized Losses
Unrealized losses related to securities for which the Company has recognized credit losses in earnings	$ 0	$ 0
Unsettled purchases or sales of Investments in fixed maturities	697,300,000	453,700,000
Other receivables	245,400,000	194,300,000
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	401,100,000	140,000,000
12 Months or Longer	$ 79,000,000	$ 103,400,000
Total	480,100,000	243,400,000
Unrealized Losses
Less Than 12 Months	(7,600,000)	(1,100,000)
12 Months or Longer	(7,400,000)	(5,100,000)
Total	$ (15,000,000)	$ (6,200,000)
States and Political Subdivisions [Member]
Fair Value
Less Than 12 Months	299,400,000	57,000,000
12 Months or Longer	$ 102,600,000	$ 124,200,000
Total	402,000,000	181,200,000
Unrealized Losses
Less Than 12 Months	(5,000,000)	(300,000)
12 Months or Longer	(3,600,000)	(2,000,000)
Total	$ (8,600,000)	$ (2,300,000)
Debt Security, Government, Non-US [Member]
Fair Value
Less Than 12 Months	4,900,000
12 Months or Longer	$ 0
Total	4,900,000
Unrealized Losses
Less Than 12 Months	(600,000)
12 Months or Longer	0
Total	$ (600,000)
Corporate Bonds and Notes [Member]
Fair Value
Less Than 12 Months	1,326,000,000	283,100,000
12 Months or Longer	$ 116,800,000	$ 208,900,000
Total	1,442,800,000	492,000,000
Unrealized Losses
Less Than 12 Months	(38,200,000)	(4,200,000)
12 Months or Longer	(10,900,000)	(6,800,000)
Total	$ (49,100,000)	$ (11,000,000)
Collateralized Loan Obligations [Member]
Fair Value
Less Than 12 Months	439,200,000	2,800,000
12 Months or Longer	$ 0	$ 2,400,000
Total	439,200,000	5,200,000
Unrealized Losses
Less Than 12 Months	(13,200,000)	0
12 Months or Longer	0	0
Total	$ (13,200,000)	$ 0
Other Mortgage and Asset-backed Securities [Member]
Fair Value
Less Than 12 Months	200,000
12 Months or Longer	$ 4,500,000
Total	4,700,000
Unrealized Losses
Less Than 12 Months	0
12 Months or Longer	(100,000)
Total	$ (100,000)
Investments in Fixed Maturities [Member]
Fair Value
Less Than 12 Months	2,470,800,000	482,900,000
12 Months or Longer	$ 302,900,000	$ 438,900,000
Total	2,773,700,000	921,800,000
Unrealized Losses
Less Than 12 Months	(64,600,000)	(5,600,000)
12 Months or Longer	(22,000,000)	(13,900,000)
Total	(86,600,000)	(19,500,000)
Unsettled purchases or sales of Investments in fixed maturities	10,500,000	5,400,000
Other receivables	$ 500,000	$ 0
Investments in Fixed Maturities [Member] | External Credit Rating, Non Investment Grade [Member]
Unrealized Losses
Percentage of unrealized loss position to amortized cost basis of available for sale security average	5.00%	4.00%
Equity Securities [Member]
Unrealized Losses
Unsettled purchases or sales of Investments in fixed maturities	$ 0
Other receivables	$ 0