Stock-based compensation - Additional Information (Detail) (USD $)	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 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	$ 12,000,000		$ 12,000,000
Employee Service Share Based Compensation Nonvested Awards Total Compensation Cost Not Yet Recognized Period For Recognition			1.5
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	166,215	210,303	166,215	210,303
Income Tax Expense Benefit	(77,893,000)	(38,100,000)	(61,701,000)	109,127,000
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 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
Share Based Compensation Arrangement By Share Based Payment Award Equity Instruments Other Than Options Grants In Period	207,237	63,689	359,427	155,945	155,945
Restricted Stock Expense	1,200,000	800,000	2,100,000	1,300,000
Employee Service Share Based Compensation Tax Benefit From Compensation Expense	300,000	200,000	500,000	300,000
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
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	100,980	18,788	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, Options, Outstanding, Intrinsic Value	600,000		600,000
Share Based Compensation Arrangement By Share Based Payment Award Equity Instruments Other Than Options Grants In Period	29,103	19,542	34,478	21,871	30,163
Restricted Stock Expense	100,000	100,000	200,000	200,000
Employee Service Share Based Compensation Tax Benefit From Compensation Expense	33,000	35,000	70,000	70,000
Performance Based Shares [Member]
Deferred Compensation Arrangement with Individual, Share-based Payments [Line Items]
Share Based Compensation Arrangement By Share Based Payment Award Award Vesting Rights			The performance shares are granted at the end of the three-year period and vest at grant date, except when the participant’s employment is terminated by the Corporation without cause. In such case, the participant would receive a pro-rata amount of shares calculated as if the Corporation would have met the performance goal for the performance period.
Share Based Compensation Arrangement By Share Based Payment Award Equity Instruments Other Than Options Grants In Period	0	0	0	0
Performance Awards [Member]
Deferred Compensation Arrangement with Individual, Share-based Payments [Line Items]
Employee Service Share Based Compensation Tax Benefit From Compensation Expense	0	0	0	0
Allocated Share Based Compensation Expense	$ 0	$ 0	$ 0	$ 0