Shareholders' equity - Issued and Outstanding Preferred Stock (Parenthetical) (Detail) (USD $) In Millions, except Share data, unless otherwise specified	1 Months Ended		9 Months Ended
	Aug. 30, 2012	May 31, 2011	Sep. 30, 2012
Class of Stock [Line Items]
Preferred stock dividend rate			9.00%
Change in the dividend rate after the respective five-year anniversary date	6.375%
Date of change in the dividend rate	Nov. 15, 2013
Preferred shares redemption date	Nov. 15, 2018
Preferred stock, shares redeemed		330,000,000
Redemption days of shares by Company		90 days
Series A [Member]
Class of Stock [Line Items]
Exercise price of each class of warrants or rights outstanding			73.86
Preferred stock, shares redeemed		370,000
Accelerated amortization of preferred stock discount as per redemption		11.2
Series D [Member]
Class of Stock [Line Items]
Preferred stock dividend rate		6.875%
Ten Year Warrant Series A Preferred [Member]
Class of Stock [Line Items]
Ten-year warrant to purchase common stock			1,218,522
Ten Year Warrant Series C Preferred [Member]
Class of Stock [Line Items]
Ten-year warrant to purchase common stock			407,542
Series C [Member]
Class of Stock [Line Items]
Exercise price of each class of warrants or rights outstanding			55.76
Preferred series A and C first five years [Member]
Class of Stock [Line Items]
Preferred stock dividend rate			5.00%