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Long-Term Debt - Additional Information (Detail) - USD ($)
$ in Thousands
9 Months Ended
Sep. 30, 2018
Mar. 31, 2018
Dec. 31, 2017
Nov. 30, 2017
Debt Instrument [Line Items]        
Document Period End Date Sep. 30, 2018      
Long-term debt including current maturities $ 250,700   $ 205,200  
Debt Instrument, Unused Borrowing Capacity, Amount $ 200,000      
Revolving Credit Facility [Member]        
Debt Instrument [Line Items]        
Long-term debt including current maturities   $ 25,000    
Debt Instrument, Interest Rate, Stated Percentage      
3.48% note, due May 31, 2038 [Member]        
Debt Instrument [Line Items]        
Long-term debt including current maturities $ 50,000      
3.25% note, due April 30, 2032 [Member]        
Debt Instrument [Line Items]        
Long-term debt including current maturities $ 70,000   $ 70,000  
Notes Payable, Other Payables [Member] | Senior Notes [Member]        
Debt Instrument [Line Items]        
Debt Instrument, Interest Rate, Stated Percentage 3.48%      
Shelf Notes [Member]        
Debt Instrument [Line Items]        
Debt Instrument, Interest Rate, Stated Percentage 3.98%      
Prudential [Member]        
Debt Instrument [Line Items]        
Senior notes $ 100,000      
Debt Instrument, Unused Borrowing Capacity, Amount 50,000      
Prudential [Member] | Notes Payable, Other Payables [Member] | Shelf Notes [Member]        
Debt Instrument [Line Items]        
Senior notes $ 150,000      
Debt Instrument, Interest Rate, Stated Percentage 3.25%      
Prudential [Member] | Shelf Notes [Member]        
Debt Instrument [Line Items]        
Senior notes $ 150,000      
MetLife [Member] | Notes Payable, Other Payables [Member] | Shelf Notes [Member]        
Debt Instrument [Line Items]        
Senior notes $ 150,000      
Maturity date term 20 years      
Debt Instrument, Unused Borrowing Capacity, Amount $ 150,000      
New York Life [Member] | Notes Payable, Other Payables [Member] | Shelf Notes [Member]        
Debt Instrument [Line Items]        
Senior notes $ 100,000      
Maturity date term 20 years      
Series A [Member] | New York Life [Member]        
Debt Instrument [Line Items]        
Senior notes       $ 50,000
Debt Instrument, Interest Rate, Stated Percentage       3.48%
Series B [Member] | New York Life [Member]        
Debt Instrument [Line Items]        
Debt Instrument, Interest Rate, Stated Percentage       3.58%