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Short-Term Borrowings and Long-Term Debt Outstanding Short Term Borrowings and Long Term Debt (Additional Information) (Detail) - USD ($)
1 Months Ended 12 Months Ended
Jan. 29, 2015
Sep. 30, 2010
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
Debt Outstanding [Line Items]          
Weighted average interest rates associated with short-term borrowings     0.59% 0.32%  
Proceeds from issuance of 3.50% Senior Notes     $ 0 $ 346,431,000 $ 0
3.50% Senior Notes          
Debt Outstanding [Line Items]          
Stated interest rate     3.50% 3.50%  
Principal value $ 350,000,000   $ 350,000,000    
Maturity     Jan. 29, 2025    
Senior Notes     $ 346,979,000 $ 346,667,000  
Deferred Finance Costs, Gross     3,000,000    
Debt Instrument, Unamortized Discount     $ 300,000    
5.375% Senior Notes          
Debt Outstanding [Line Items]          
Stated interest rate     5.375% 5.375%  
Principal value   $ 350,000,000 $ 350,000,000    
Maturity     Sep. 15, 2020    
Proceeds from issuance of 3.50% Senior Notes $ 346,400,000 $ 345,000,000      
Senior Notes     $ 347,586,000 $ 347,016,000  
6.05% Subordinated Notes          
Debt Outstanding [Line Items]          
Fair value of the interest rate swap associated with the notes     $ 800,000 $ 2,800,000  
Stated interest rate     6.05% 6.05%  
Principal value     $ 45,964,000    
Maturity     Jun. 01, 2017    
6.05% Subordinated Notes | Interest rate swaps          
Debt Outstanding [Line Items]          
Fair value of the interest rate swap associated with the notes     $ 800,000 $ 2,800,000  
7.0% Junior Subordinated Debentures          
Debt Outstanding [Line Items]          
Stated interest rate     7.00% 7.00%  
Principal value     $ 50,000,000    
Maturity     Oct. 15, 2033