Shareholders' equity - Issued and Outstanding Preferred Stock (Parenthetical) (Detail) (USD $) In Millions, except Share data, unless otherwise specified	1 Months Ended										1 Months Ended				12 Months Ended		1 Months Ended
	Aug. 31, 2012	May 31, 2011	Dec. 31, 2012	Dec. 31, 2012 Series A Fixed Rate Cumulative Perpetual Preferred Stock [Member]	Dec. 31, 2011 Series A Fixed Rate Cumulative Perpetual Preferred Stock [Member]	Dec. 31, 2012 Series C Fixed Rate Cumulative Perpetual Preferred Stock [Member]	Dec. 31, 2011 Series C Fixed Rate Cumulative Perpetual Preferred Stock [Member]	Dec. 31, 2012 Series D Fixed Rate Non-cumulative Perpetual Preferred Stock [Member]	Dec. 31, 2011 Series D Fixed Rate Non-cumulative Perpetual Preferred Stock [Member]	Dec. 31, 2012 Ten Year Warrant Series A Preferred [Member]	May 31, 2011 Series A [Member]	Dec. 31, 2012 Series A [Member]	Dec. 31, 2012 Ten Year Warrant Series C Preferred [Member]	Dec. 31, 2012 Series C [Member]	Dec. 31, 2012 Preferred series A and C first five years [Member]	Dec. 31, 2012 Preferred series A and C after five years [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%	9.00%	6.875%
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											370,000
Accelerated amortization of preferred stock discount as per redemption											$ 11.2
Redemption days of shares by Company		90 days