Shareholders' equity - Issued and Outstanding Preferred Stock (Parenthetical) (Detail) (USD $)	1 Months Ended														3 Months Ended			1 Months Ended
	Aug. 31, 2012	May 31, 2011	Mar. 31, 2013	Dec. 31, 2012	Mar. 31, 2013 Series A Fixed Rate Cumulative Perpetual Preferred Stock [Member]	Dec. 31, 2012 Series A Fixed Rate Cumulative Perpetual Preferred Stock [Member]	Mar. 31, 2013 Series C Fixed Rate Cumulative Perpetual Preferred Stock [Member]	Dec. 31, 2012 Series C Fixed Rate Cumulative Perpetual Preferred Stock [Member]	Mar. 31, 2013 Series D Fixed Rate Non-Cumulative Perpetual Preferred Stock [Member]	Dec. 31, 2012 Series D Fixed Rate Non-Cumulative Perpetual Preferred Stock [Member]	Mar. 31, 2013 Ten Year Warrant Series A Preferred [Member]	Mar. 31, 2013 Series A [Member]	Mar. 31, 2013 Ten Year Warrant Series C Preferred [Member]	Mar. 31, 2013 Series C [Member]	Mar. 31, 2013 Preferred series A and C first five years [Member]	Mar. 31, 2013 Preferred series A and C after five years [Member]	Mar. 31, 2013 Series C warrants [Member]	May 31, 2011 Series D [Member]
Class of Stock [Line Items]
Liquidation preference					$ 1,000	$ 1,000	$ 1,000	$ 1,000	$ 10,000	$ 10,000
Ten-year warrant to purchase common stock											1,218,522		407,542
Exercise price of each class of warrants or rights outstanding			26.50									73.86		55.76
Preferred stock dividend rate															5.00%	6.375%		6.875%
Date of change in the dividend rate	Nov. 14, 2013
Preferred shares redemption date	Nov. 15, 2018
Issuance of common shares			128,952,578	128,176,912													186,589
Redemption days of shares by Company		90 days