Stock-Based Compensation - Additional Information (Detail) (USD $)	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2011	Jun. 30, 2010	Jun. 30, 2011	Jun. 30, 2010	Dec. 31, 2010
Deferred Compensation Arrangement with Individual, Share-based Payments [Line Items]
Stock option, maximum option term (in years)	10 years
Total unrecognized compensation cost related to non-vested restricted stock awards and performance shares to members of management	$ 5,500,000		$ 5,500,000
Total unrecognized compensation cost related to non-vested restricted stock awards and performance shares to members of management, period of recognition (in years)	2
Stock option, exercisable	2,103,034	2,530,137	2,103,034	2,530,137
Additional income tax expense	(38,100,000)	27,237,000	109,127,000	17,962,000
Employee Stock Option
Deferred Compensation Arrangement with Individual, Share-based Payments [Line Items]
Stock-Based Compensation, 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 (RSUs)
Deferred Compensation Arrangement with Individual, Share-based Payments [Line Items]
Stock-Based Compensation, 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.
Restricted Stock Units (RSUs) | Management
Deferred Compensation Arrangement with Individual, Share-based Payments [Line Items]
Stock granted	636,889	563,043	1,559,463	1,525,416	1,525,416
Restricted stock expense	800,000	(200,000)	1,300,000	100,000
Incentive award, tax benefit	200,000	(56,000)	300,000	71,000
Fair Market Value of Restricted Stock	74,000
Additional income tax expense	3,000
Restricted Stock Units (RSUs) | Management | TARP Interim Final Rule
Deferred Compensation Arrangement with Individual, Share-based Payments [Line Items]
Stock granted	187,880	360,527	1,110,454	1,246,755
Restricted Stock Units (RSUs) | Management | At Grant Date
Deferred Compensation Arrangement with Individual, Share-based Payments [Line Items]
Fair Market Value of Restricted Stock	90,000
Restricted Stock Units (RSUs) | Director
Deferred Compensation Arrangement with Individual, Share-based Payments [Line Items]
Stock granted	195,423	207,261	218,707	242,394	305,898
Restricted stock expense	100,000	100,000	200,000	300,000
Incentive award, tax benefit	35,000	60,000	70,000	100,000
Fair Market Value of Restricted Stock	600,000
Performance Based Shares
Deferred Compensation Arrangement with Individual, Share-based Payments [Line Items]
Stock-Based Compensation, 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.
Stock granted			0	12,426
Performance Awards
Deferred Compensation Arrangement with Individual, Share-based Payments [Line Items]
Incentive award, tax benefit				60,000
Performance share expense				$ 100,000