Comprehensive income - Accumulated Other Comprehensive Income (Loss), Net Consisted of Unrealized Gains (Losses) (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance - December 31, 2012	$ (240,264)	$ (356,441)
Net gain (loss) during period	14,311	25,183
Balance - March 31, 2013	(225,953)	(331,258)
Defined Benefit Plans [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance - December 31, 2012	(276,771)
Net gain (loss) during period	5,164
Balance - March 31, 2013	(271,607)
Other [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance - December 31, 2012	(284)
Net gain (loss) during period	(932)
Balance - March 31, 2013	(1,216)
Investment Securities [Member] | Investment Securities With OTTI [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance - December 31, 2012	(55,790)
Net gain (loss) during period	20,891
Balance - March 31, 2013	(34,899)
Investment Securities [Member] | Investment Securities All Other [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance - December 31, 2012	92,581
Net gain (loss) during period	(10,812)
Balance - March 31, 2013	$ 81,769