Investments - Amortized Cost and Estimated Fair Value of investments (Details) - USD ($)	Dec. 31, 2016	Dec. 31, 2015
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 4,846,800,000	$ 4,560,700,000
Gross Unrealized Gains	324,600,000	341,600,000
Gross Unrealized Losses	(46,500,000)	(50,000,000)
Fair Value	5,124,900,000	4,852,300,000
Accrued Expenses and Other Liabilities	453,200,000	472,400,000
Other Receivables	198,600,000	193,200,000
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
Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Accrued Expenses and Other Liabilities	0	0
Other Receivables	200,000	0
Investments in Fixed Maturities [Member]
Schedule of Available-for-sale Securities [Line Items]
Accrued Expenses and Other Liabilities	100,000	5,600,000
Other Receivables	2,700,000	0
US Government and Government Agencies and Authorities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	321,200,000	298,000,000
Gross Unrealized Gains	22,300,000	26,200,000
Gross Unrealized Losses	(7,200,000)	(3,600,000)
Fair Value	336,300,000	320,600,000
States and Political Subdivisions [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	1,640,600,000	1,513,700,000
Gross Unrealized Gains	88,400,000	111,600,000
Gross Unrealized Losses	(14,100,000)	(2,700,000)
Fair Value	1,714,900,000	1,622,600,000
Foreign Government Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	3,500,000
Gross Unrealized Gains	0
Gross Unrealized Losses	(100,000)
Fair Value	3,400,000
Corporate Bonds and Notes [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	2,758,900,000	2,651,500,000
Gross Unrealized Gains	209,900,000	202,000,000
Gross Unrealized Losses	(24,000,000)	(40,700,000)
Fair Value	2,944,800,000	2,812,800,000
Redeemable Preferred Stocks [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	500,000	3,700,000
Gross Unrealized Gains	100,000	100,000
Gross Unrealized Losses	0	0
Fair Value	600,000	3,800,000
Collateralized Loan Obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	121,200,000	90,000,000
Gross Unrealized Gains	2,700,000	300,000
Gross Unrealized Losses	(1,100,000)	(3,000,000)
Fair Value	122,800,000	87,300,000
Other Mortgage and Asset-backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	900,000	3,800,000
Gross Unrealized Gains	1,200,000	1,400,000
Gross Unrealized Losses	0	0
Fair Value	2,100,000	5,200,000
Collateralized Debt Obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair Value	900,000	3,800,000
Non-governmental Residential Mortgage-backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair Value	$ 1,200,000	1,300,000
Other Asset Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair Value		$ 100,000