Investments - Amortized Cost and Estimated Fair Value of investments (Details) - USD ($)	Dec. 31, 2015	Dec. 31, 2014
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 4,560,700,000	$ 4,341,700,000
Gross Unrealized Gains	341,600,000	455,600,000
Gross Unrealized Losses	(50,000,000)	(19,700,000)
Fair Value	4,852,300,000	4,777,600,000
Accrued Expenses and Other Liabilities	472,400,000	409,600,000
Other Receivables	193,200,000	187,000,000
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
Investments in Fixed Maturities [Member]
Schedule of Available-for-sale Securities [Line Items]
Accrued Expenses and Other Liabilities	5,600,000	0
Other Receivables		0
US Government and Government Agencies and Authorities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	298,000,000	315,200,000
Gross Unrealized Gains	26,200,000	32,300,000
Gross Unrealized Losses	(3,600,000)	(2,000,000)
Fair Value	320,600,000	345,500,000
States and Political Subdivisions [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	1,513,700,000	1,352,500,000
Gross Unrealized Gains	111,600,000	126,400,000
Gross Unrealized Losses	(2,700,000)	(1,800,000)
Fair Value	1,622,600,000	1,477,100,000
Corporate Bonds and Notes [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	2,651,500,000	2,599,300,000
Gross Unrealized Gains	202,000,000	294,300,000
Gross Unrealized Losses	(40,700,000)	(15,100,000)
Fair Value	2,812,800,000	2,878,500,000
Redeemable Preferred Stocks [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	3,700,000	5,900,000
Gross Unrealized Gains	100,000	800,000
Gross Unrealized Losses	0	0
Fair Value	3,800,000	6,700,000
Collateralized Loan Obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	90,000,000	64,900,000
Gross Unrealized Gains	300,000	300,000
Gross Unrealized Losses	(3,000,000)	(800,000)
Fair Value	87,300,000	64,400,000
Other Mortgage and Asset-backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	3,800,000	3,900,000
Gross Unrealized Gains	1,400,000	1,500,000
Gross Unrealized Losses	0	0
Fair Value	5,200,000	5,400,000
Collateralized Debt Obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair Value	3,800,000	3,900,000
Non-governmental Residential Mortgage-backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair Value	1,300,000	1,300,000
Other Asset Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair Value	$ 100,000	$ 200,000