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Debt (Details) - USD ($)
3 Months Ended 6 Months Ended
Jun. 30, 2018
Jun. 30, 2017
Jun. 30, 2018
Jun. 30, 2017
Mar. 31, 2018
Dec. 31, 2017
Debt Instrument [Line Items]            
Long-term Debt, Gross $ 841,800,000   $ 841,800,000     $ 592,300,000
Equity Securities at Fair Value 514,900,000   514,900,000      
Debt Instrument, Collateral Amount 16,700,000   16,700,000      
Interest Expense 7,900,000 $ 8,200,000 15,900,000 $ 19,100,000    
Interest Paid, Including Capitalized Interest, Operating and Investing Activities 2,800,000 $ 10,800,000 15,300,000 $ 19,100,000    
Secured Debt [Member]            
Debt Instrument [Line Items]            
Debt Instrument, Face Amount 250,000,000   250,000,000      
Long-term Debt, Gross 249,400,000   249,400,000     0
Senior Notes [Member] | Senior Notes, 4.35 Percent Due February 15, 2025 [Member]            
Debt Instrument [Line Items]            
Long-term Debt, Gross 448,200,000   448,200,000     448,100,000
Subordinated Debt [Member] | Subordinated Debenture, 7.375 Percent Due 2054 [Member]            
Debt Instrument [Line Items]            
Long-term Debt, Gross 144,200,000   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     0
Revolving Credit Facility [Member]            
Debt Instrument [Line Items]            
Line of Credit Facility, Maximum Borrowing Capacity 300,000,000   300,000,000     225,000,000
Revolving Credit Facility [Member] | Credit facility and Secured Loan [Member]            
Debt Instrument [Line Items]            
Line of Credit Facility, Additional Borrowing Capacity, Available 100,000,000   100,000,000      
Federal Home Loan Bank of Dallas [Member]            
Debt Instrument [Line Items]            
Federal Home Loan Bank, Advances, Branch of FHLB Bank, Amount of Advances           0
Equity Securities at Fair Value 3,300,000   3,300,000     3,300,000
Federal Home Loan Bank, Advances, Branch of FHLB Bank, Amount of Advances 55,000,000   55,000,000      
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   10,000,000     0
Equity Securities at Fair Value 1,300,000   1,300,000     400,000
Federal Home Loan Bank, Advances, Branch of FHLB Bank, Amount of Advances 55,000,000   55,000,000   $ 10,000,000  
Federal Home Loan Bank Borrowings, Fair Value Disclosure $ 10,000,000   $ 10,000,000     $ 0