Comprehensive income - Accumulated Other Comprehensive Income (Loss), Net Consisted of Unrealized Gains (Losses) (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance	$ (356,441)	$ (205,220)	$ (335,997)
Net gain (loss) during period	116,177	(151,221)	130,777
Ending Balance	(240,264)	(356,441)	(205,220)
Cash Flow Hedges [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance	112	393	674
Net gain (loss) during period	(112)	(281)	(281)
Ending Balance		112	393
Foreign Currency Translation Adjustment [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance	(803)
Net gain (loss) during period	519	(803)
Ending Balance	(284)	(803)
Defined Benefit Plans [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance	(277,716)	(120,892)	(117,046)
Net gain (loss) during period	945	(156,824)	(3,846)
Ending Balance	(276,771)	(277,716)	(120,892)
Investment Securities [Member] | Investment Securities With OTTI [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance	(84,029)	(87,053)	(76,772)
Net gain (loss) during period	28,239	3,024	(10,281)
Ending Balance	(55,790)	(84,029)	(87,053)
Investment Securities [Member] | Investment Securities All other [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance	5,995	2,332	(142,853)
Net gain (loss) during period	86,586	3,663	145,185
Ending Balance	$ 92,581	$ 5,995	$ 2,332