Pension and other postretirement benefits - reconciliation of changes in level three pension assets measured at fair value on recurring basis (Detail) (Hedge Fund Of Funds, USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013
Hedge Fund Of Funds
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Balance at beginning of year	$ 51,654	$ 48,198
Unrealized gain on assets	2,698	3,456
Balance at end of year	$ 54,352	$ 51,654