Comprehensive income - Accumulated Other Comprehensive Income (Loss), Net Consisted of Unrealized Gains (Losses) (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance			$ (356,441)	$ (205,220)
Net gain (loss) during period	47,774	36,236	126,366	12,687
Ending Balance	(230,075)	(192,533)	(230,075)	(192,533)
Cash flow hedges [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance			112	393
Net gain (loss) during period			(112)	(211)
Ending Balance		182		182
Foreign currency translation adjustment [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance			(803)
Net gain (loss) during period			351	(503)
Ending Balance	(452)	(503)	(452)	(503)
Defined benefit plans [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance			(277,716)	(120,892)
Net gain (loss) during period			14,196	6,431
Ending Balance	(263,520)	(114,461)	(263,520)	(114,461)
Investment securities [Member] | Investment securities With OTTI [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance			(84,029)	(87,053)
Net gain (loss) during period			16,267	3,016
Ending Balance	(67,762)	(84,037)	(67,762)	(84,037)
Investment securities [Member] | Investment securities All other [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance			5,995	2,332
Net gain (loss) during period			95,664	3,954
Ending Balance	$ 101,659	$ 6,286	$ 101,659	$ 6,286