Stock-based compensation - Additional Information (Detail) (USD $)	3 Months Ended		12 Months Ended
	Mar. 31, 2012	Mar. 31, 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	$ 7,000,000
Employee Service Share Based Compensation Nonvested Awards Total Compensation Cost Not Yet Recognized Period For Recognition	1.5
Stock Option Plan Expense	0	0
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Intrinsic Value
Stock option, exercisable	1,682,158	2,121,618
Income Tax Expense Benefit	16,192,000	147,227,000
Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercises in Period, Total Intrinsic Value
Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercisable, Intrinsic Value
Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercises in Period	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 | 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 . The five-year vesting part is accelerated at termination of employment after attaining 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	1,521,904	1,559,463	1,559,463
Restricted Stock Expense	900,000	500,000
Employee Service Share Based Compensation Tax Benefit From Compensation Expense	200,000	100,000
Share Based Compensation Arrangement By Share Based Payment Award Equity Instruments Other Than Options Vested In Period Total Fair Value	0
Income Tax Expense Benefit	0
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
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	1,521,904	1,559,463
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	86,000
Share Based Compensation Arrangement By Share Based Payment Award Equity Instruments Other Than Options Grants In Period	53,750	23,284	305,898
Restricted Stock Expense	100,000	100,000
Employee Service Share Based Compensation Tax Benefit From Compensation Expense	35,000	35,000
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
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
Allocated Share Based Compensation Expense	$ 0	$ 0