Shareholders' equity - Issued and Outstanding Preferred Stock (Parenthetical) (Detail) (USD $)	3 Months Ended		3 Months Ended		3 Months Ended		3 Months Ended		3 Months Ended					1 Months Ended	12 Months Ended
	Mar. 31, 2014 Series A Fixed Rate Cumulative Perpetual Preferred Stock [Member]	Dec. 31, 2013 Series A Fixed Rate Cumulative Perpetual Preferred Stock [Member]	Mar. 31, 2014 Series C Fixed Rate Cumulative Perpetual Preferred Stock [Member]	Dec. 31, 2013 Series C Fixed Rate Cumulative Perpetual Preferred Stock [Member]	Mar. 31, 2014 Series D Fixed Rate Non-Cumulative Perpetual Preferred Stock [Member]	Dec. 31, 2013 Series D Fixed Rate Non-Cumulative Perpetual Preferred Stock [Member]	Mar. 31, 2014 Series E Fixed-to-Floating Rate Non-Cumulative Perpetual Preferred Stock [Member]	Dec. 31, 2013 Series E Fixed-to-Floating Rate Non-Cumulative Perpetual Preferred Stock [Member]	Mar. 31, 2014 Dividend Paid Rate Up To November Fourteen Two Thousand And Thirteen [Member] Series A Fixed Rate Cumulative Perpetual Preferred Stock [Member]	Mar. 31, 2014 Dividend Paid Rate Up To November Fourteen Two Thousand And Thirteen [Member] Series C Fixed Rate Cumulative Perpetual Preferred Stock [Member]	Mar. 31, 2014 Dividend Paid Rate Thereafter November Fourteen Two Thousand And Thirteen [Member] Series A Fixed Rate Cumulative Perpetual Preferred Stock [Member]	Mar. 31, 2014 Dividend Paid Rate Thereafter November Fourteen Two Thousand And Thirteen [Member] Series C Fixed Rate Cumulative Perpetual Preferred Stock [Member]	Mar. 31, 2014 Warrant [Member] Series A Fixed Rate Cumulative Perpetual Preferred Stock [Member]	Mar. 31, 2013 Warrant [Member] Series C Fixed Rate Cumulative Perpetual Preferred Stock [Member]	Dec. 31, 2013 Cash Less Exercise [Member] Warrant [Member] Series A Fixed Rate Cumulative Perpetual Preferred Stock [Member]
Class of Stock [Line Items]
Liquidation preference	$ 1,000	$ 1,000	$ 1,000	$ 1,000	$ 10,000	$ 10,000	$ 1,000	$ 1,000
Cashless warrants exercised	1,218,522		407,542												69,127
Exercise price of each class of warrants or rights outstanding	$ 73.86		$ 55.76
Preferred stock dividend rate					6.875%		6.45%		5.00%	5.00%	6.375%	6.375%
Date of change in the dividend rate	Nov. 14, 2013		Nov. 14, 2013				Feb. 14, 2024
Preferred shares redemption date	Nov. 15, 2018		Nov. 15, 2018		Jun. 15, 2016		Feb. 15, 2024
Issuance of common shares														186,589	25,427
Warrants and rights outstanding													1,149,395
Warrants, year of expiration													2018
Preferred Stock, Redemption Feature, Redemption Term					90 days		90 days
London Interbank offered rate plus basis points							3.61%