Shareholders' equity - Issued and Outstanding Preferred Stock (Detail) (USD $) In Thousands, except Share data, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Class of Stock [Line Items]
Carrying value	$ 874,627	$ 872,500
Series A Fixed Rate Cumulative Perpetual Preferred Stock [Member]
Class of Stock [Line Items]
Preferred stock, shares issued	230,000
Carrying value	227,711	226,965
Series C Fixed Rate Cumulative Perpetual Preferred Stock [Member]
Class of Stock [Line Items]
Preferred stock, shares issued	151,500
Carrying value	146,916	145,535
Series D Fixed Rate Non-Cumulative Perpetual Preferred Stock [Member]
Class of Stock [Line Items]
Preferred stock, shares issued	50,000
Carrying value	$ 500,000	$ 500,000