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Comprehensive Income - Accumulated Other Comprehensive Income (Loss), Net (Detail) - USD ($)
$ in Thousands
3 Months Ended 6 Months Ended
Jun. 30, 2018
Jun. 30, 2017
Jun. 30, 2018
Jun. 30, 2017
Accumulated Other Comprehensive Income (Loss) [Line Items]        
Beginning balance     $ 16,250,819 $ 16,486,622
Cumulative effect of change in accounting principle — equity securities     (16,853)  
Net gain (loss) during period $ (34,698) $ 21,486 (137,274) 24,555
Ending balance 15,577,885 16,283,537 15,577,885 16,283,537
Investment Securities [Member]        
Accumulated Other Comprehensive Income (Loss) [Line Items]        
Beginning balance     (44,150)  
Cumulative effect of change in accounting principle — equity securities     (16,853)  
Net gain (loss) during period     (137,417)  
Ending balance (198,420)   (198,420)  
Defined Benefit Plans [Member]        
Accumulated Other Comprehensive Income (Loss) [Line Items]        
Beginning balance     (304,546)  
Net gain (loss) during period     14,298  
Ending balance (290,248)   (290,248)  
Accumulated Other Adjustment [Member]        
Accumulated Other Comprehensive Income (Loss) [Line Items]        
Beginning balance     (15,118)  
Net gain (loss) during period     (14,155)  
Ending balance (29,273)   (29,273)  
Accumulated Other Comprehensive Income (Loss), Net [Member]        
Accumulated Other Comprehensive Income (Loss) [Line Items]        
Beginning balance     (363,814) (294,636)
Cumulative effect of change in accounting principle — equity securities     (16,853)  
Net gain (loss) during period     (137,274)  
Ending balance $ (517,941) $ (270,081) $ (517,941) $ (270,081)