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Fair Values of Financial Instruments - Additional Information (Detail)
9 Months Ended 12 Months Ended
Sep. 30, 2018
Dec. 31, 2017
Jan. 19, 2018
Jun. 14, 2012
Oct. 05, 2010
3.95% Senior Notes Due 2028          
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]          
Debt interest rate 3.95%   3.95%    
Current market rates used in measuring fair value 4.82%        
4.97% Series A Senior Notes, Due October 5th 2020          
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]          
Debt interest rate         4.97%
Current market rates used in measuring fair value 4.56% 4.46%      
4.27% Series B Senior Notes, Due June 14th 2022          
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]          
Debt interest rate       4.27%  
Current market rates used in measuring fair value 4.65% 4.64%