Segment information - Information about Company's Segments (Detail) (USD $)	3 Months Ended		12 Months Ended
	Mar. 31, 2013	Mar. 31, 2012	Dec. 31, 2012
Segment Reporting Information [Line Items]
Total revenues	$ 1,088,932,000	$ 997,112,000
Net income (loss)	274,113,000	206,463,000
Average total assets	81,913,000,000	78,026,000,000	79,983,000,000
Business Banking [Member]
Segment Reporting Information [Line Items]
Total revenues	105,418,000	111,976,000
Inter-segment revenues	1,194,000	1,092,000
Net income (loss)	32,561,000	35,333,000
Average total assets	4,980,000,000	5,206,000,000	4,909,000,000
Commercial Banking [Member]
Segment Reporting Information [Line Items]
Total revenues	249,850,000	237,462,000
Inter-segment revenues	1,350,000	1,540,000
Net income (loss)	107,387,000	103,096,000
Average total assets	21,272,000,000	19,158,000,000	19,946,000,000
Commercial Real Estate [Member]
Segment Reporting Information [Line Items]
Total revenues	165,293,000	153,544,000
Inter-segment revenues	1,552,000	433,000
Net income (loss)	76,508,000	68,631,000
Average total assets	17,054,000,000	16,316,000,000	16,437,000,000
Discretionary Portfolio [Member]
Segment Reporting Information [Line Items]
Total revenues	12,041,000	(1,160,000)
Inter-segment revenues	(8,601,000)	(14,781,000)
Net income (loss)	1,846,000	(8,241,000)
Average total assets	16,585,000,000	15,804,000,000	16,583,000,000
Residential Mortgage Banking [Member]
Segment Reporting Information [Line Items]
Total revenues	119,899,000	93,042,000
Inter-segment revenues	18,698,000	34,441,000
Net income (loss)	34,361,000	23,517,000
Average total assets	2,847,000,000	2,142,000,000	2,451,000,000
Retail Banking [Member]
Segment Reporting Information [Line Items]
Total revenues	291,185,000	306,848,000
Inter-segment revenues	3,257,000	2,892,000
Net income (loss)	52,350,000	48,657,000
Average total assets	11,391,000,000	11,856,000,000	11,705,000,000
All Other [Member]
Segment Reporting Information [Line Items]
Total revenues	145,246,000	95,400,000
Inter-segment revenues	(17,450,000)	(25,617,000)
Net income (loss)	(30,900,000)	(64,530,000)
Average total assets	$ 7,784,000,000	$ 7,544,000,000	$ 7,952,000,000