Net accumulated other comprehensive income (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012
Comprehensive Income [Line Items]
Balance at January 1, 2013	$ 20,258	$ 20,006
Other comprehensive income before reclassification, net of tax	(1,831)	(664)
Reclassifications from accumulated other comprehensive income, net of tax	33	44
Current period change in other comprehensive income, net of tax	(1,798)	(620)
Balance at March 31, 2013	18,460	19,386
Balance at January 1, 2013	(8,946)	(6,758)
Other comprehensive income before reclassification, net of tax	0	0
Reclassifications from accumulated other comprehensive income, net of tax	218	210
Current period change in other comprehensive income, net of tax	218	210
Balance at March 31, 2013	(8,728)	(6,548)
Balance at January 1, 2013	11,312	13,248
Other comprehensive income before reclassification, net of tax	(1,831)	(664)
Reclassifications from accumulated other comprehensive income, net of tax	251	254
Current period change in other comprehensive income, net of tax	(1,580)	(410)
Balance at March 31, 2013	$ 9,732	$ 12,838