Schedule of Actual Weighted Average Shares Used in Calculating Earnings Per Share (Parenthetical) (Detail) In Millions, unless otherwise specified	1 Months Ended	3 Months Ended
	Apr. 30, 2012	Mar. 31, 2012
Schedule of Weighted Average Number of Diluted Shares Outstanding [Line Items]
Shares included in dilutive weighted average share amounts related to equity units		4.1	[1]
Number of shares to be issued for equity units	5.8

[1] For the three months ended March 31, 2012, 4.1 million shares were included in the dilutive weighted average share amount related to the Equity Units. The number of shares that was issued on April 30, 2012, related to the final settlement of the Equity Units, was approximately 5.8 million. This reflects the effect from the exchange of Equity Units discussed in Note 9 and taking into account all previously paid dividends, including the dividend paid in the first quarter 2012.