Shareholders' equity - Issued and Outstanding Preferred Stock (Detail) (USD $) In Thousands, except Share data, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Class of Stock [Line Items]
Carrying value	$ 872,500	$ 864,585
Series A Fixed Rate Cumulative Perpetual Preferred Stock [Member]
Class of Stock [Line Items]
Preferred stock, shares issued	230,000	230,000
Carrying value	226,965	224,277
Series C Fixed Rate Cumulative Perpetual Preferred Stock [Member]
Class of Stock [Line Items]
Preferred stock, shares issued	151,500	151,500
Carrying value	145,535	140,308
Series D Fixed Rate Non-cumulative Perpetual Preferred Stock [Member]
Class of Stock [Line Items]
Preferred stock, shares issued	50,000	50,000
Carrying value	$ 500,000	$ 500,000