Debt (Details) - USD ($)	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Debt Instrument [Line Items]
Long-term Debt	$ 592,300,000		$ 592,300,000
Equity Securities at Fair Value	568,200,000
Interest Expense	8,000,000	$ 10,900,000
Interest Paid	12,500,000	$ 8,300,000
Senior Notes [Member] | Senior Notes, 4.35 Percent Due February 15, 2025 [Member]
Debt Instrument [Line Items]
Long-term Debt	448,100,000		448,100,000
Subordinated Debt [Member] | Subordinated Debenture, 7.375 Percent Due 2054 [Member]
Debt Instrument [Line Items]
Long-term Debt	144,200,000		144,200,000
Notes Payable under Revolving Credit Agreement [Member]
Debt Instrument [Line Items]
Line of Credit Facility, Amount Outstanding	0		0
Revolving Credit Facility [Member]
Debt Instrument [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity	225,000,000		225,000,000
Federal Home Loan Bank of Dallas [Member]
Debt Instrument [Line Items]
Federal Home Loan Bank, Advances, Branch of FHLB Bank, Amount of Advances	0		0
Equity Securities at Fair Value	3,300,000		3,300,000
Federal Home Loan Bank of Chicago [Member]
Debt Instrument [Line Items]
Federal Home Loan Bank, Advances, Branch of FHLB Bank, Amount of Advances	10,000,000		0
Equity Securities at Fair Value	$ 400,000		$ 400,000