Activity of Awarded Deferred Common Stock Units (Detail) (Deferred Common Stock Units, USD $)	12 Months Ended
	Dec. 31, 2011
Deferred Common Stock Units
Number of shares
Balance at beginning of year	103,256
Granted	105,527
Balance at end of year	208,783
Weighted-Average Grant Date Fair Value
Balance at beginning of year	$ 26.76
Granted	$ 8.31
Balance at end of year	$ 17.34