Stock-based compensation - Additional Information (Detail) (USD $)	3 Months Ended		12 Months Ended
	Mar. 31, 2015	Mar. 31, 2014	Dec. 31, 2014
Deferred Compensation Arrangement with Individual, Share-based Payments [Line Items]
Share Based Compensation Arrangement By Share Based Payment Award Award Vesting Period	10 years 0 months 0 days
Employee Service Share Based Compensation Nonvested Awards Total Compensation Cost Not Yet Recognized	$ 11,700,000
Employee Service Share Based Compensation Nonvested Awards Total Compensation Cost Not Yet Recognized Period For Recognition	0 years 0 months 511 days
Stock Option Plan Expense	0	0
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Intrinsic Value	0	0
Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercises in Period	0		0
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.
Adjustments To Additional Paid In Capital Sharebased Compensation And Exercise Of Stock Options	69,000
Restricted Stock Units RSU 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.  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 the earlier of 55 years of age and 10 years of service or 60 years of age and 5 years of service.  The four year vesting part is accelerated at termination of employment after attaining the earlier of 55 years of age and 10 years of service or 60 years of age and 5 years of service. 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	105,783	365,831
Restricted Stock Expense	2,000,000	1,300,000
Employee Service Share Based Compensation Tax Benefit From Compensation Expense	300,000	400,000
Share Based Compensation Arrangement By Share Based Payment Award Equity Instruments Other Than Options Vested In Period Total Fair Value	3,500,000
Adjustments To Additional Paid In Capital Sharebased Compensation And Exercise Of Stock Options	69,000
Nonrecognized Windfall	100,000
Windfall Net Of Shortfall	200,000
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	2,900,000
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	2,643	3,085	23,135
Restricted Stock Expense	100,000	100,000
Employee Service Share Based Compensation Tax Benefit From Compensation Expense	16,000	49,000
Share Based Compensation Arrangement By Share Based Payment Award Equity Instruments Other Than Options Vested In Period Total Fair Value	$ 85,000