Activity of Awarded Restricted Stock Units (Detail) (USD $)	12 Months Ended
	Dec. 31, 2012 Restricted Stock Units (RSUs)		Dec. 31, 2012 Performance Based Restricted Stock Units (PBRSUs)	Dec. 31, 2012 Retirement Eligible Employees, Equity Award Restricted Stock Units (RSUs)	Dec. 31, 2011 Retirement Eligible Employees, Equity Award Restricted Stock Units (RSUs)
Number of shares
Balance at beginning of year (in shares)	99,826	[1]	0	61,344	78,864
Granted (in shares)	108,157		216,441
Vested (in shares)	(105,773)
Forfeited (in shares)	(10,239)		(15,617)
Balance at end of year (in shares)	91,971	[1]	200,824	61,344	78,864
Weighted-Average Grant Date Fair Value
Balance at beginning of year (in dollars per share)	$ 18.10	[1]	$ 0.00
Granted (in dollars per share)	$ 5.67		$ 6.80
Vested (in dollars per share)	$ 13.09
Forfeited (in dollars per share)	$ 9.20		$ 12.80
Balance at end of year (in dollars per share)	$ 10.08	[1]	$ 6.67

[1] Excludes 61,344 units at the beginning of the year and 78,864 units at the end of year that have vested due to retirement eligibility, but have not yet been settled or converted to common shares.