Segment information - Information about Company's Segments (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
Operating Statistics [Line Items]
Total Revenue	$ 1,108,455	$ 985,101	$ 3,145,200	$ 2,956,183
Net income (loss)	293,462	183,108	733,305	711,739
Business Banking [Member]
Operating Statistics [Line Items]
Total Revenue	114,181	110,627	335,475	314,416
Inter-segment revenues	1,011	957	3,202	2,890
Net income (loss)	38,411	31,237	110,829	84,121
Commercial Banking [Member]
Operating Statistics [Line Items]
Total Revenue	255,498	230,023	741,714	672,199
Inter-segment revenues	1,561	1,318	4,832	3,674
Net income (loss)	112,654	93,783	315,373	277,225
Commercial Real Estate [Member]
Operating Statistics [Line Items]
Total Revenue	169,806	152,878	491,433	412,250
Inter-segment revenues	5,031	410	5,860	1,214
Net income (loss)	75,896	65,566	225,025	179,634
Discretionary Portfolio [Member]
Operating Statistics [Line Items]
Total Revenue	1,805	16,437	(11,162)	171,303
Inter-segment revenues	(25,593)	(3,968)		(16,174)
Net income (loss)	(5,039)	2,077		76,566
Residential Mortgage Banking [Member]
Operating Statistics [Line Items]
Total Revenue	138,662	67,757	341,699	183,910
Inter-segment revenues	28,454	25,012	100,350	44,018
Net income (loss)	44,727	10,805	94,238	21,556
Retail Banking [Member]
Operating Statistics [Line Items]
Total Revenue	318,567	314,113	939,409	920,645
Inter-segment revenues	2,841	2,583	8,931	8,537
Net income (loss)	58,441	43,947	165,289	151,318
All Other [Member]
Operating Statistics [Line Items]
Total Revenue	109,936	93,266	306,632	281,460
Inter-segment revenues	(13,305)	(26,312)		(44,159)
Net income (loss)	$ (31,628)	$ (64,307)		$ (78,681)