Stock-based compensation - Additional Information (Detail) (USD $)	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011	Dec. 31, 2011
Deferred Compensation Arrangement with Individual, Share-based Payments [Line Items]
Employee Service Share Based Compensation Nonvested Awards Total Compensation Cost Not Yet Recognized	$ 13,000,000		$ 13,000,000
Employee Service Share Based Compensation Nonvested Awards Total Compensation Cost Not Yet Recognized Period For Recognition			1.1
Stock Option Plan Expense	0	0	0	0
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Intrinsic Value	0	0	0	0
Stock option, exercisable	165,194	207,813	165,194	207,813
Income Tax Expense Benefit	15,384,000	5,537,000	(46,317,000)	114,664,000
Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercises in Period, Total Intrinsic Value	0	0	0	0
Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercisable, Intrinsic Value	0	0	0	0
Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercises in Period	0	0	0	0	0
Employee Stock Option[Member]
Deferred Compensation Arrangement with Individual, Share-based Payments [Line Items]
Share Based Compensation Arrangement By Share Based Payment Award Award Vesting Period			10 years
Share Based Compensation Arrangement By Share Based Payment Award Award Vesting Rights			all options granted are 20% exercisable after the first year and an additional 20% is exercisable after each subsequent year, subject to an acceleration clause at termination of employment due to retirement
Restricted Stock Units RSU Member | T R P Interim Final Rule [Member]
Deferred Compensation Arrangement with Individual, Share-based Payments [Line Items]
Share Based Compensation Arrangement By Share Based Payment Award Award Vesting Rights			The restricted shares granted consistent with the requirements of the Troubled Asset Relief Program (“TARP”) Interim Final Rule vest in two years from grant date.
Restricted Stock Units RSU Member | Management Member
Deferred Compensation Arrangement with Individual, Share-based Payments [Line Items]
Share Based Compensation Arrangement By Share Based Payment Award Equity Instruments Other Than Options Grants In Period	0	0	359,427	155,945	155,945
Restricted Stock Expense	1,100,000	300,000	3,200,000	1,500,000
Employee Service Share Based Compensation Tax Benefit From Compensation Expense	300,000	49,000	800,000	400,000
Share Based Compensation Arrangement By Share Based Payment Award Equity Instruments Other Than Options Vested In Period Total Fair Value			1,600,000
Income Tax Expense Benefit			300,000
Share Based Compensation Arrangement By Share Based Payment Award Award Vesting Rights			the compensation cost associated with the shares of restricted stock is determined based on a two-prong vesting schedule. The first part is vested ratably over five years commencing at the date of grant and the second part is vested at termination of employment after attainment of 55 years of age and 10 years of service
Restricted Stock Units RSU Member | Management Member | Share Vesting On Grant Date [Member]
Deferred Compensation Arrangement with Individual, Share-based Payments [Line Items]
Share Based Compensation Arrangement By Share Based Payment Award Equity Instruments Other Than Options Vested In Period Total Fair Value			2,700,000
Restricted Stock Units RSU Member | Management Member | T R P Interim Final Rule [Member]
Deferred Compensation Arrangement with Individual, Share-based Payments [Line Items]
Share Based Compensation Arrangement By Share Based Payment Award Equity Instruments Other Than Options Grants In Period	0	0	253,170	111,045
Restricted Stock Units RSU Member | Director Member
Deferred Compensation Arrangement with Individual, Share-based Payments [Line Items]
Share Based Compensation Arrangement By Share Based Payment Award Equity Instruments Other Than Options Grants In Period	3,322	2,792	37,800	24,662	30,163
Restricted Stock Expense	100,000	100,000	300,000	300,000
Employee Service Share Based Compensation Tax Benefit From Compensation Expense	32,000	35,000	100,000	100,000
Share Based Compensation Arrangement By Share Based Payment Award Equity Instruments Other Than Options Vested In Period Total Fair Value			$ 600,000