Pension plans and other postretirement benefits - Fair Values of Company's Pension Plan Assets by Asset Category (Parenthetical) (Detail) (USD $)	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Interest and dividend receivable on plan assets	$ 4,633,000	$ 4,406,000
Investment grade bonds of U.S. issuers [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Investment in mutual funds	40.00%	50.00%
High-yielding bonds [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Investment in mutual funds	60.00%	50.00%