Income taxes - Deferred Tax Assets (Liabilities) (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Income Tax Rate Reconciliation [Line Items]
Losses on loans and other assets	$ 809,033	$ 896,219	$ 550,970
Postretirement and other employee benefits	34,517	39,992	28,135
Incentive compensation plans	50,067	49,183	27,388
Interest on loans	72,278	46,965	43,563
Retirement benefits	91,980	147,997	42,422
Stock-based compensation	69,874	78,014	70,641
Unrealized investment losses		50,528	54,557
Depreciation and amortization	12,130	9,563	13,332
Other	103,027	99,012	51,768
Gross deferred tax assets	1,242,906	1,417,473	882,776
Leasing transactions	(291,524)	(294,150)	(294,510)
Unrealized investment gains	(23,574)
Capitalized servicing rights	(20,348)	(17,603)	(14,739)
Interest on subordinated note exchange	(8,794)	(11,275)	(13,534)
Other	(61,410)	(48,803)	(36,080)
Gross deferred tax liabilities	(405,650)	(371,831)	(358,863)
Net deferred tax asset	$ 837,256	$ 1,045,642	$ 523,913