| 
           Significant Accounting Policies - Effects of Prior Period Errors in Condensed Consolidated Statement of Cash Flows (Detail) (USD $) 
        In Thousands, unless otherwise specified  | 
        3 Months Ended | 6 Months Ended | ||
|---|---|---|---|---|
| 
           Jun. 30, 2013 
         | 
        
           Jun. 30, 2012 
         | 
        
           Jun. 30, 2013 
         | 
        
           Jun. 30, 2012 
         | 
      |
| Net income | $ 143,162 | $ 131,951 | $ 262,035 | $ 239,882 | 
| Deferred income taxes | (7,939) | (8,476) | ||
| Income taxes | (12,546) | 20,661 | ||
| 
           As Previously Reported [Member] 
         | 
        ||||
| Net income | 133,368 | 241,530 | ||
| Deferred income taxes | (4,896) | |||
| Income taxes | $ 15,433 | |||
| X | ||||||||||
| 
             
- Definition 
          Amount of deferred income tax expense (benefit) pertaining to income (loss) from continuing operations. Reference 1: http://www.xbrl.org/2003/role/presentationRef 
  | 
        
| X | ||||||||||
| 
             
- Definition 
          The increase (decrease) during the period in the amount due for taxes based on the reporting entity's earnings or attributable to the entity's income earning process (business presence) within a given jurisdiction. Reference 1: http://www.xbrl.org/2003/role/presentationRef 
  | 
        
| X | ||||||||||
| 
             
- Definition 
          The portion of profit or loss for the period, net of income taxes, which is attributable to the parent. Reference 1: http://www.xbrl.org/2003/role/presentationRef 
  |