Summary of Shares of Outstanding Restricted Stock Units and Restricted Stock Awards Activity (Parenthetical) (Detail) In Millions, unless otherwise specified	9 Months Ended
	Sep. 30, 2012 Times
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of remaining optional early delivery dates	1
Restricted Stock Units (RSUs)
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Restricted stock units vested but not yet delivered	1.5
Period 1
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Optional early delivery date	2013