Shareholders' equity - Issued and Outstanding Preferred Stock (Detail) (USD $) In Thousands, except Share data, unless otherwise specified	Mar. 31, 2014	Dec. 31, 2013
Class of Stock [Line Items]
Carrying value	$ 1,231,500	$ 881,500
Series A Fixed Rate Cumulative Perpetual Preferred Stock [Member]
Class of Stock [Line Items]
Preferred stock, shares issued	230,000
Preferred stock, shares outstanding	230,000
Carrying value	230,000	230,000
Series C Fixed Rate Cumulative Perpetual Preferred Stock [Member]
Class of Stock [Line Items]
Preferred stock, shares issued	151,500
Preferred stock, shares outstanding	151,500
Carrying value	151,500	151,500
Series D Fixed Rate Non-Cumulative Perpetual Preferred Stock [Member]
Class of Stock [Line Items]
Preferred stock, shares issued	50,000
Preferred stock, shares outstanding	50,000
Carrying value	500,000	500,000
Series E Fixed-to-Floating Rate Non-Cumulative Perpetual Preferred Stock [Member]
Class of Stock [Line Items]
Preferred stock, shares issued	350,000
Preferred stock, shares outstanding	350,000
Carrying value	$ 350,000