Stockholders' equity (deficit) (Details) (USD $) In Millions, except Share data, unless otherwise specified	3 Months Ended		1 Months Ended						1 Months Ended			3 Months Ended
	Jan. 31, 2012	Jan. 31, 2011	Sep. 30, 2011 Maximum [Member]	Nov. 30, 2011 Accelerated Share Repurchases Program [Member]	Oct. 31, 2011 Accelerated Share Repurchases Program [Member]	Oct. 31, 2011 October 2011 [Member]	Oct. 31, 2011 October 2011 [Member] Accelerated Share Repurchases Program [Member]	Oct. 31, 2011 November 2011 [Member]	Nov. 30, 2011 November 2011 [Member] Accelerated Share Repurchases Program [Member]	Jan. 31, 2012 Open Market Share Repurchase Program [Member]	Oct. 31, 2011 Open Market Share Repurchase Program [Member]	Jan. 31, 2012 Open Market Share Repurchase Program [Member]	Apr. 30, 2012 Open Market Share Repurchase Program [Member] Repurchase of Equity [Member]
Class of Stock [Line Items]
Stock Repurchase Program, Authorized Amount			$ 175		$ 100
Payments for Repurchase of Common Stock	70	0			100
Treasury Stock, Shares, Acquired				2,542,609			2,380,952			1,905,600
Accelerated Share Repurchases, Settlement Payment or Receipt						80		20
Accelerated Share Repurchases, Initial Price Paid Per Share						$ 33.60
Stock Repurchase Program, Remaining Authorized Repurchase Amount					$ 20						$ 75	$ 70	$ 5
Treasury Stock, Additional Shares Acquired									161,657