Activity of Awarded Restricted Stock Units (Detail) (Restricted Stock Units (RSUs), USD $)	12 Months Ended
	Dec. 31, 2011
Restricted Stock Units (RSUs)
Number of shares
Balance at beginning of year	72,215	[1]
Granted	78,763
Vested	(51,152)
Balance at end of year	99,826	[1]
Weighted-Average Grant Date Fair Value
Balance at beginning of year	$ 18.77	[1]
Granted	$ 17.14
Vested	$ 18.38
Balance at end of year	$ 18.10	[1]

[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.