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Deferred Compensation - Additional information (Detail) - USD ($)
shares in Millions
 3 Months Ended 9 Months Ended
Sep. 30, 2018
Dec. 31, 2017
Sep. 30, 2018
Mar. 31, 2018
Sep. 30, 2017
Mar. 31, 2017
Sep. 30, 2018
Sep. 30, 2017
Investments And Employee Deferred Compensation Plan [Line Items]                
Deferred Equity Participation Plan, distributions to key executives, age             Age 62 (or the one-year anniversary of the date of the grant for participants over the age of 61 as of the grant date) or upon or after their actual retirement.  
Deferred Equity Participation Sub-plans, distributions to key executives, age             Age 65  
Deferred Equity Participation Sub-plans, distributions requisite service description             We annually make awards under sub-plans of the DEPP for certain production staff, which generally provide for vesting and/or distributions no sooner than five years from the date of awards, although certain awards vest and/or distribute after the earlier of fifteen years or the participant reaching age 65.  
Deferred Equity Participation Plan (DEPP) [Member]                
Investments And Employee Deferred Compensation Plan [Line Items]                
Awards approved by committee, value     $ 0 $ 11,500,000 $ 0 $ 14,000,000    
Charge to compensation expenses related to awards     2,400,000   2,600,000   $ 6,700,000 $ 7,000,000
Cash and equity awards with aggregate fair value vested and distributed to participants             0 0
Unearned deferred compensation, value $ 60,700,000 $ 54,700,000            
Unearned deferred compensation, shares 2.8 2.6            
Total intrinsic value of unvested equity based awards $ 210,100,000 $ 166,000,000            
Deferred Cash Participation Plan (DCPP) [Member]                
Investments And Employee Deferred Compensation Plan [Line Items]                
Awards approved by committee, value       5,600,000   5,100,000    
Charge to compensation expenses related to awards     900,000   700,000   2,700,000 1,800,000
Cash and equity awards with aggregate fair value vested and distributed to participants             0 0
Deferred Equity Participation Plan Sub Plans [Member]                
Investments And Employee Deferred Compensation Plan [Line Items]                
Awards approved by committee, value     0 $ 900,000 0 $ 4,000,000    
Charge to compensation expenses related to awards     $ 600,000   $ 500,000   1,700,000 $ 1,400,000
Cash and equity awards with aggregate fair value vested and distributed to participants             $ 0