Shareholders' equity - Issued and Outstanding Preferred Stock (Detail) (USD $) In Thousands, except Share data, unless otherwise specified	Sep. 30, 2012	Dec. 31, 2011
Class of Stock [Line Items]
Liquidation preference	$ 1,000	$ 1,000
Preferred stock, shares issued	381,500	381,500
Carrying value	$ 870,416	$ 864,585
Series A Fixed Rate Cumulative Perpetual Preferred Stock [Member]
Class of Stock [Line Items]
Liquidation preference	$ 1,000	$ 1,000
Preferred stock, shares issued	230,000
Carrying value	226,232	224,277
Series C Fixed Rate Cumulative Perpetual Preferred Stock [Member]
Class of Stock [Line Items]
Liquidation preference	$ 1,000	$ 1,000
Preferred stock, shares issued	151,500
Carrying value	144,184	140,308
Series D Fixed Rate Non-Cumulative Perpetual Preferred Stock [Member]
Class of Stock [Line Items]
Liquidation preference	$ 10,000	$ 10,000
Preferred stock, shares issued	50,000
Carrying value	$ 500,000	$ 500,000