Common Stock - Additional Information (Detail) (USD $) In Thousands, except Share data, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Shareholders Equity [Line Items]
Common shares outstanding	4,914,130	4,977,633
Treasury stock repurchases, shares	22,700,000
Treasury stock repurchases, value	$ 4,754,000
Repurchased shares during the period, shares	285,495	1,017,588	644,562
Repurchased shares during the period, value	$ 227,300	$ 689,300	$ 417,100
Treasury stock reissued, shares	7,062,000
Reissued shares during the period, shares	221,992	333,380	359,765