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Debt (Narrative) (Details) - USD ($)
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Jun. 30, 2006
Debt Instrument [Line Items]        
Debt Instrument, Unamortized Discount (Premium) and Debt Issuance Costs, Net $ 20,405,000 $ 22,218,000    
Undrawn Outstanding Letters Of Credit In Favor Of Third Parties 120,100,000 189,400,000    
Undrawn Outstanding Letters Of Credit Between Subsidiaries 1,492,200,000 $ 1,010,800,000    
Debt Instrument, Face Amount       $ 850,000,000.0
Line of Credit Facility, Maximum Borrowing Capacity $ 1,266,500,000.0      
Debt, Weighted Average Interest Rate 5.24% 5.16%    
Long-term Debt, Gross $ 2,808,770,000 $ 3,110,853,000    
Long-term debt 2,788,365,000 3,088,635,000    
Long-term Debt, Maturities, Repayments of Principal in Year Four 403,040,000      
Line of Credit Facility, Commitment Fee Amount 10,500,000 7,900,000 $ 10,900,000  
Senior Notes Due 2026 [Member]        
Debt Instrument [Line Items]        
Debt Instrument, Unamortized Discount (Premium) and Debt Issuance Costs, Net $ 8,800,000      
Debt Instrument, Interest Rate, Stated Percentage 3.95%      
Debt Instrument, Maturity Date Dec. 31, 2026      
Debt Instrument, Face Amount $ 400,000,000      
Proceeds from Issuance of Long-term Debt and Capital Securities, Net 791,200,000      
Long-term Debt, Gross $ 399,987,000 399,985,000    
Subordinated Debentures Due 2056 [Member]        
Debt Instrument [Line Items]        
Debt Instrument, Interest Rate, Stated Percentage 5.75%      
Debt Instrument, Maturity Date Dec. 31, 2056      
Debt Instrument, Face Amount $ 400,000,000.0      
Long-term Debt, Gross $ 400,000,000 400,000,000    
Junior Subordinated Debentures Due 2065 [Member]        
Debt Instrument [Line Items]        
Debt Instrument, Maturity Date Dec. 31, 2065      
Debt Instrument, Face Amount $ 400,000,000      
Long-term Debt, Gross $ 318,740,000 318,737,000    
Senior Notes Due 2021 [Member]        
Debt Instrument [Line Items]        
Debt Instrument, Interest Rate, Stated Percentage 5.00%      
Debt Instrument, Maturity Date Dec. 31, 2021      
Debt Instrument, Face Amount $ 400,000,000      
Long-term Debt, Gross $ 399,245,000 399,025,000    
Subordinated Debentures Due 2042 [Member]        
Debt Instrument [Line Items]        
Debt Instrument, Interest Rate, Stated Percentage 6.20%      
Debt Instrument, Maturity Date Dec. 31, 2042      
Debt Instrument, Face Amount $ 400,000,000      
Long-term Debt, Gross $ 400,000,000 400,000,000    
Senior Notes Due 2023 [Member]        
Debt Instrument [Line Items]        
Debt Instrument, Interest Rate, Stated Percentage 4.70%      
Debt Instrument, Maturity Date Dec. 31, 2023      
Debt Instrument, Face Amount $ 400,000,000      
Long-term Debt, Gross $ 399,138,000 398,986,000    
Notes Payable to Banks [Member]        
Debt Instrument [Line Items]        
Debt Instrument, Interest Rate, Stated Percentage 4.09%      
Debt Instrument, Maturity Date Dec. 31, 2039      
Debt Instrument, Face Amount $ 100,000,000      
Long-term Debt, Gross $ 91,787,000 94,370,000    
Senior Notes Due 2019 [Member]        
Debt Instrument [Line Items]        
Debt Instrument, Interest Rate, Stated Percentage 6.45%      
Debt Instrument, Maturity Date Dec. 31, 2019      
Debt Instrument, Face Amount $ 400,000,000      
Long-term Debt, Gross $ 399,873,000 399,805,000    
Senior Notes Due 2017 [Member]        
Debt Instrument [Line Items]        
Debt Instrument, Interest Rate, Stated Percentage 5.625%      
Debt Instrument, Maturity Date Dec. 31, 2017      
Debt Instrument, Face Amount $ 300,000,000      
Long-term Debt, Gross 0 299,945,000    
Syndicated Credit Facility $850 Million [Member]        
Debt Instrument [Line Items]        
Line of Credit Facility, Maximum Borrowing Capacity 850,000,000      
Long-term Line of Credit [1] $ 96,564,000 $ 96,095,000    
[1] Represents issued but undrawn letters of credit. There was no cash borrowed for the periods presented.