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Debt (Details) - USD ($)
3 Months Ended 9 Months Ended
Jun. 12, 2017
Sep. 30, 2017
Sep. 30, 2016
Sep. 30, 2017
Sep. 30, 2016
Dec. 31, 2016
Feb. 24, 2015
Debt Instrument [Line Items]              
Long-term Debt   $ 592,200,000   $ 592,200,000   $ 751,600,000  
Loss on Cash Flow Hedge Ineffectiveness       1,100,000      
Interest Expense, Other   100,000          
Cash Flow Hedge Derivative Instrument Assets at Fair Value           1,600,000  
Cash Flow Hedge Gain (Loss) to be Reclassified within Twelve Months       (400,000)      
Interest Expense   7,800,000 $ 11,100,000 26,900,000 $ 33,400,000    
Interest Paid   12,700,000 $ 8,500,000 31,800,000 $ 30,500,000    
Senior Notes [Member] | Senior Notes, 6 Percent Due May 15, 2017 [Member]              
Debt Instrument [Line Items]              
Long-term Debt   0   0   359,800,000  
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   247,700,000  
Debt Instrument, Face Amount $ 200,000,000           $ 250,000,000.0
Debt Instrument, Interest Rate, Stated Percentage             4.35%
Proceeds from Debt, Net of Issuance Costs $ 200,200,000            
Debt Instrument, Interest Rate, Effective Percentage 4.16%            
Subordinated Debt [Member] | Subordinated Debenture, 7.375 Percent Due 2054 [Member]              
Debt Instrument [Line Items]              
Long-term Debt   144,100,000   144,100,000   144,100,000  
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   225,000,000   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   0  
Federal Home Loan Bank of Chicago [Member]              
Debt Instrument [Line Items]              
Federal Home Loan Bank, Advances, Branch of FHLB Bank, Amount of Advances   0   0   $ 0  
Cash Flow Hedging [Member] | Designated as Hedging Instrument [Member]              
Debt Instrument [Line Items]              
Effective portion of the loss on the derivative instrument upon discontinuance   $ 4,500,000   4,500,000      
Interest Expense, Other       $ 100,000