Activity of Awarded Deferred Common Stock Units (Detail) (Deferred Common Stock Units, USD $)	12 Months Ended
	Dec. 31, 2012
Deferred Common Stock Units
Number of shares
Balance at beginning of year (in shares)	208,783
Granted (in shares)	29,295
Converted (in shares)	(35,202)
Balance at end of year (in shares)	202,876
Weighted-Average Grant Date Fair Value
Balance at beginning of year (in dollars per share)	$ 17.34
Granted (in dollars per share)	$ 5.38
Converted (in dollars per share)	$ 18.95
Balance at end of year (in dollars per share)	$ 15.33