Pension plans and other postretirement benefits - Changes in Level 3 Pension Plan Assets Measured at Fair Value on Recurring Basis (Detail) (USD $) In Thousands, unless otherwise specified							12 Months Ended			12 Months Ended			12 Months Ended			12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2012 Significant Unobservable Inputs (Level 3) [Member]	Dec. 31, 2011 Significant Unobservable Inputs (Level 3) [Member]	Dec. 31, 2012 Other Private real estate [Member]	Dec. 31, 2011 Other Private real estate [Member]	Dec. 31, 2012 Other Private real estate [Member] Significant Unobservable Inputs (Level 3) [Member]	Dec. 31, 2012 Other Private equity [Member]	Dec. 31, 2011 Other Private equity [Member]	Dec. 31, 2012 Other Private equity [Member] Significant Unobservable Inputs (Level 3) [Member]	Dec. 31, 2012 Other Hedge funds [Member]	Dec. 31, 2011 Other Hedge funds [Member]	Dec. 31, 2012 Other Hedge funds [Member] Significant Unobservable Inputs (Level 3) [Member]	Dec. 31, 2012 Other Securities [Member]	Dec. 31, 2011 Other Securities [Member]	Dec. 31, 2012 Other Securities [Member] Significant Unobservable Inputs (Level 3) [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Fair value of plan assets at beginning of year	$ 1,404,138	$ 1,113,566	$ 41,459	$ 59,794	$ 4,603	$ 4,677	$ 4,677	$ 5,347	$ 10,190	$ 10,190	$ 41,509	$ 44,927	$ 44,927	$ 61,874	$ 77,351	$ 59,794
Sales										(4,892)			(15,000)			(19,892)
Total Realized/Unrealized Gains (Losses)							(74)			49			1,582			1,557
Fair value of plan assets at end of year	$ 1,404,138	$ 1,113,566	$ 41,459	$ 59,794	$ 4,603	$ 4,677	$ 4,603	$ 5,347	$ 10,190	$ 5,347	$ 41,509	$ 44,927	$ 31,509	$ 61,874	$ 77,351	$ 41,459