Activity of Awarded Restricted Stock Units (Parenthetical) (Detail) (Restricted Stock Units (RSUs))	Dec. 31, 2011		Dec. 31, 2010
Deferred Compensation Arrangement with Individual, Share-based Payments [Line Items]
Outstanding shares	99,826	[1]	72,215	[1]
Retirement Eligible Employees, Equity Award
Deferred Compensation Arrangement with Individual, Share-based Payments [Line Items]
Outstanding shares	61,344

[1] Excludes 61,344 units at both the beginning and end of year that have vested due to retirement eligibility, but have not yet been settled or converted to common shares.