Stock-based compensation - Additional Information (Detail) (USD $)	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013	Dec. 31, 2013
Deferred Compensation Arrangement with Individual, Share-based Payments [Line Items]
Employee Service Share Based Compensation Nonvested Awards Total Compensation Cost Not Yet Recognized	$ 8,600,000		$ 8,600,000
Employee Service Share Based Compensation Nonvested Awards Total Compensation Cost Not Yet Recognized Period For Recognition			0 years 0 months 730 days
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
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
Adjustments To Additional Paid In Capital Sharebased Compensation And Exercise Of Stock Options			417,000
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 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
Deferred Compensation Arrangement with Individual, Share-based Payments [Line Items]
Nonrecognized Windfall	800,000
Windfall Net Of shortfall	1,200,000
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 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	235,112	229,131	229,131
Restricted Stock Expense	1,600,000	1,400,000	4,700,000	3,900,000
Employee Service Share Based Compensation Tax Benefit From Compensation Expense	200,000	400,000	700,000	1,200,000
Share Based Compensation Arrangement By Share Based Payment Award Equity Instruments Other Than Options Vested In Period Total Fair Value			8,800,000
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. The five-year vesting part is accelerated at termination of employment after attaining 55 years of age and 10 years of service. The vesting schedule for restricted shares granted on 2014 was modified as follows, the first part ratably over four years commencing at the date of the grant and the second part is vested at termination of employment after attainment of 55 years of age and 10 years of service or 60 years of age and 5 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	0		5,600,000
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	56,549	61,245	162,332	165,304
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	2,318	1,669	21,051	18,885	20,930
Restricted Stock Expense	100,000	100,000	400,000	400,000
Employee Service Share Based Compensation Tax Benefit From Compensation Expense	14,000	46,000	43,000	100,000
Share Based Compensation Arrangement By Share Based Payment Award Equity Instruments Other Than Options Vested In Period Total Fair Value			$ 600,000