Shareholders' equity (Details) (USD $)	1 Months Ended	6 Months Ended
	May 31, 2011	Jun. 30, 2012	Dec. 31, 2011
Issued and outstanding preferred stock
Preferred stock, shares issued		381,500	381,500
Carrying value		$ 868,433,000	$ 864,585,000
Shareholders Equity (Textual) [Abstract]
Preferred stock, par value		$ 1.00	$ 1.00
Liquidation preference		$ 1,000	$ 1,000
Preferred stock, shares authorized		1,000,000	1,000,000
Preferred stock, shares redeemed	330,000,000
Limitation by U.S. Treasury on maximum amount of dividend to be paid per common stock quarterly		$ 0.70
Redemption days of shares by Company	90 days
Series A Fixed Rate Cumulative Perpetual Preferred Stock [Member]
Issued and outstanding preferred stock
Preferred stock, shares issued		230,000
Carrying value		225,570,000	224,277,000
Shareholders Equity (Textual) [Abstract]
Liquidation preference		$ 1,000	$ 1,000
Series C Fixed Rate Cumulative Perpetual Preferred Stock [Member]
Issued and outstanding preferred stock
Preferred stock, shares issued		151,500
Carrying value		142,863,000	140,308,000
Shareholders Equity (Textual) [Abstract]
Liquidation preference		$ 1,000	$ 1,000
Series D Fixed Rate Non Cumulative Perpetual Preferred Stock [Member]
Issued and outstanding preferred stock
Preferred stock, shares issued		50,000
Carrying value		500,000,000	500,000,000
Shareholders Equity (Textual) [Abstract]
Liquidation preference		$ 10,000	$ 10,000
Ten Year Warrant Series A Preferred [Member]
Shareholders Equity (Textual) [Abstract]
Ten-year warrant to purchase common stock		1,218,522
Ten Year Warrant Series C Preferred [Member]
Shareholders Equity (Textual) [Abstract]
Ten-year warrant to purchase common stock		407,542
Ten Year Warrant Series D Preferred [Member]
Shareholders Equity (Textual) [Abstract]
Ten-year warrant to purchase common stock		95,383	95,383
Exercise price of each class of warrants or rights outstanding		518.96	518.96
Series A [Member]
Shareholders Equity (Textual) [Abstract]
Exercise price of each class of warrants or rights outstanding		73.86
Preferred stock, shares redeemed	370,000
Accelerated amortization of preferred stock discount as per redemption	11,200,000
Series C [Member]
Shareholders Equity (Textual) [Abstract]
Exercise price of each class of warrants or rights outstanding		55.76
Series D [Member]
Shareholders Equity (Textual) [Abstract]
Preferred stock dividend rate	6.875%
Preferred series A and C first five years [Member]
Shareholders Equity (Textual) [Abstract]
Preferred stock dividend rate		5.00%
Preferred series A and C after five years [Member]
Shareholders Equity (Textual) [Abstract]
Preferred stock dividend rate		9.00%