Reclassifications out of Accumulated Other Comprehensive Income Loss (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012
Amounts Reclassified From Accumulated Other Comprehensive Income (Loss) [Line Items]
Amortization of service cost	$ 1,935	$ 1,747
Total before tax	1,935	1,747
Provision for income taxes	(662)	(624)
Total net of tax	1,273	1,123
Reclassification adjustments	1,872	1,925
Total before tax	1,872	1,925
Provision for income taxes	(682)	(704)
Total net of tax	$ 1,190	$ 1,221