Equity - Additional Information (Detail) (USD $) Share data in Millions, unless otherwise specified	9 Months Ended		1 Months Ended		12 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Nov. 30, 2011 Accelerated Share Repurchase Program [Member]	May 31, 2011 Accelerated Share Repurchase Program [Member]	Dec. 31, 2011 Accelerated Share Repurchase Program [Member]
Equity [Line Items]
Stock repurchase program, authorized amount	$ 1,000,000,000
Stock repurchase program, period in force	1 year 6 months 29 days
Common stock repurchase program, approval date	May 03, 2011
Stock repurchase program, authorized amount		$ 500,000,000			$ 500,000,000
Treasury stock, shares acquired			9.3	19.9