Investments - Amortized Cost and Estimated Fair Value of investments (Details) - USD ($)	Dec. 31, 2018	Dec. 31, 2017
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 6,284,500,000	$ 5,021,600,000
Gross Unrealized Gains	226,300,000	380,600,000
Gross Unrealized Losses	(86,600,000)	(19,500,000)
Fair Value	6,424,200,000	5,382,700,000
Accrued Expenses and Other Liabilities	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
Equity Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Accrued Expenses and Other Liabilities	0
Other Receivables	0
Investments in Fixed Maturities [Member]
Debt Securities, Available-for-sale [Line Items]
Accrued Expenses and Other Liabilities	10,500,000	5,400,000
Other Receivables	500,000	0
US Government and Government Agencies and Authorities [Member]
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	865,900,000	542,700,000
Gross Unrealized Gains	14,800,000	19,600,000
Gross Unrealized Losses	(15,000,000)	(6,200,000)
Fair Value	865,700,000	556,100,000
States and Political Subdivisions [Member]
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	1,553,700,000	1,595,500,000
Gross Unrealized Gains	74,000,000	108,600,000
Gross Unrealized Losses	(8,600,000)	(2,300,000)
Fair Value	1,619,100,000	1,701,800,000
Debt Security, Government, Non-US [Member]
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	6,500,000	3,000,000
Gross Unrealized Gains	0	200,000
Gross Unrealized Losses	(600,000)	0
Fair Value	5,900,000	3,200,000
Corporate Bonds and Notes [Member]
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	3,307,800,000	2,745,800,000
Gross Unrealized Gains	135,100,000	245,800,000
Gross Unrealized Losses	(49,100,000)	(11,000,000)
Fair Value	3,393,800,000	2,980,600,000
Redeemable Preferred Stocks [Member]
Debt Securities, Available-for-sale [Line Items]
Amortized Cost		100,000
Gross Unrealized Gains		0
Gross Unrealized Losses		0
Fair Value		100,000
Collateralized Loan Obligations [Member]
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	535,700,000	134,100,000
Gross Unrealized Gains	1,500,000	5,700,000
Gross Unrealized Losses	(13,200,000)	0
Fair Value	524,000,000	139,800,000
Other Mortgage and Asset-backed Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	14,900,000	400,000
Gross Unrealized Gains	900,000	700,000
Gross Unrealized Losses	(100,000)	0
Fair Value	$ 15,700,000	$ 1,100,000