Summary of Stock Option Activity under Plan (Detail) (USD $) In Thousands, except Share data, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Options outstanding
Options outstanding, beginning of period	438,394	484,694	495,283
Granted
Exercised
Forfeited	(4,200)	(13,300)	(6,190)
Expired	(22,400)	(33,000)	(4,399)
Options outstanding, end of period	411,794	438,394	484,694
Options exercisable, end of period	378,004	378,710	396,115
Options unvested, end of period	33,790	59,684	88,579
Options outstanding, weighted Average Exercise Price
Options outstanding, beginning of period	$ 38.76	$ 39.43	$ 39.61
Granted
Exercised
Forfeited	$ 35.54	$ 37.98	$ 40.85
Expired	$ 34.25	$ 48.85	$ 57.76
Options outstanding, end of period	$ 39.04	$ 38.76	$ 39.43
Options exercisable, end of period	$ 41.00	$ 41.66	$ 43.21
Options unvested, end of period	$ 17.17	$ 20.36	$ 22.50
Options outstanding
Options outstanding, end of period	2 years 9 months 18 days	3 years 7 months 6 days	3 years 9 months 18 days
Options exercisable, end of period	2 years 6 months	3 years 1 month 6 days	3 years 4 months 24 days
Options unvested, end of period	5 years 4 months 24 days	6 years 1 month 6 days	6 years 10 months 24 days
Options outstanding, end of period, aggregate intrinsic value
Options outstanding, end of period	$ 766	$ 1,143	$ 880
Options exercisable, end of period	459	457	176
Options unvested, end of period	$ 307	$ 686	$ 704