Segment information - Information about Company's Segments (Detail) (USD $)	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Segment Reporting Information [Line Items]
Net interest income	$ 2,598,516,000	$ 2,389,756,000	$ 2,267,526,000
Noninterest income	1,667,270,000	1,582,912,000	1,108,100,000
Total income	4,265,786,000	3,972,668,000	3,375,626,000
Provision for credit losses	204,000,000	270,000,000	368,000,000
Amortization of core deposit and other intangible assets	60,631,000	61,617,000	58,103,000
Depreciation and other amortization	143,930,000	136,163,000	124,259,000
Other noninterest expense	2,304,699,000	2,280,288,000	1,732,475,000
Income (loss) before taxes	1,552,526,000	1,224,600,000	1,092,789,000
Income tax expense (benefit)	523,028,000	365,121,000	356,628,000
Net income (loss)	1,029,498,000	859,479,000	736,161,000
Average total assets	79,983,000,000	73,977,000,000	68,380,000,000
Capital expenditures	92,000,000	70,000,000	70,000,000
Residential Mortgage Banking [Member]
Segment Reporting Information [Line Items]
Net interest income	78,058,000	63,951,000	71,599,000
Noninterest income	402,211,000	184,249,000	195,540,000
Total income	480,269,000	248,200,000	267,139,000
Provision for credit losses	17,169,000	36,375,000	49,110,000
Depreciation and other amortization	46,902,000	44,349,000	46,171,000
Other noninterest expense	195,604,000	151,945,000	160,131,000
Income (loss) before taxes	220,594,000	15,531,000	11,727,000
Income tax expense (benefit)	85,671,000	2,403,000	587,000
Net income (loss)	134,923,000	13,128,000	11,140,000
Average total assets	2,451,000,000	1,958,000,000	2,217,000,000
Capital expenditures	1,000,000		1,000,000
Retail Banking [Member]
Segment Reporting Information [Line Items]
Net interest income	902,906,000	851,332,000	839,828,000
Noninterest income	349,571,000	366,042,000	380,015,000
Total income	1,252,477,000	1,217,374,000	1,219,843,000
Provision for credit losses	95,345,000	101,679,000	109,978,000
Depreciation and other amortization	32,734,000	33,713,000	31,350,000
Other noninterest expense	751,916,000	744,905,000	698,540,000
Income (loss) before taxes	372,482,000	337,077,000	379,975,000
Income tax expense (benefit)	151,616,000	137,161,000	154,680,000
Net income (loss)	220,866,000	199,916,000	225,295,000
Average total assets	11,705,000,000	11,940,000,000	12,079,000,000
Capital expenditures	15,000,000	25,000,000	33,000,000
All Other [Member]
Segment Reporting Information [Line Items]
Net interest income	(80,894,000)	(46,458,000)	50,967,000
Noninterest income	501,390,000	489,863,000	168,917,000
Total income	420,496,000	443,405,000	219,884,000
Provision for credit losses	4,917,000	(61,349,000)	(14,797,000)
Amortization of core deposit and other intangible assets	60,631,000	61,617,000	58,103,000
Depreciation and other amortization	50,536,000	45,495,000	34,838,000
Other noninterest expense	693,046,000	771,534,000	333,525,000
Income (loss) before taxes	(388,634,000)	(373,892,000)	(191,785,000)
Income tax expense (benefit)	(207,887,000)	(228,970,000)	(114,257,000)
Net income (loss)	(180,747,000)	(144,922,000)	(77,528,000)
Average total assets	7,952,000,000	8,042,000,000	5,896,000,000
Capital expenditures	76,000,000	45,000,000	31,000,000
Business Banking [Member]
Segment Reporting Information [Line Items]
Net interest income	347,067,000	334,811,000	315,407,000
Noninterest income	103,283,000	95,429,000	95,443,000
Total income	450,350,000	430,240,000	410,850,000
Provision for credit losses	22,245,000	45,689,000	74,443,000
Amortization of core deposit and other intangible assets
Depreciation and other amortization	122,000	140,000	276,000
Other noninterest expense	179,428,000	177,172,000	169,878,000
Income (loss) before taxes	248,555,000	207,239,000	166,253,000
Income tax expense (benefit)	101,484,000	84,532,000	67,687,000
Net income (loss)	147,071,000	122,707,000	98,566,000
Average total assets	4,909,000,000	5,192,000,000	4,843,000,000
Capital expenditures			1,000,000
Commercial Banking [Member]
Segment Reporting Information [Line Items]
Net interest income	753,678,000	655,407,000	582,231,000
Noninterest income	253,808,000	249,150,000	217,368,000
Total income	1,007,486,000	904,557,000	799,599,000
Provision for credit losses	15,781,000	29,823,000	47,675,000
Amortization of core deposit and other intangible assets
Depreciation and other amortization	567,000	553,000	552,000
Other noninterest expense	262,820,000	248,428,000	219,471,000
Income (loss) before taxes	728,318,000	625,753,000	531,901,000
Income tax expense (benefit)	296,894,000	256,311,000	217,734,000
Net income (loss)	431,424,000	369,442,000	314,167,000
Average total assets	19,946,000,000	17,650,000,000	15,461,000,000
Capital expenditures			3,000,000
Commercial Real Estate [Member]
Segment Reporting Information [Line Items]
Net interest income	531,398,000	467,683,000	384,147,000
Noninterest income	133,120,000	99,409,000	85,200,000
Total income	664,518,000	567,092,000	469,347,000
Provision for credit losses	4,238,000	58,474,000	45,781,000
Amortization of core deposit and other intangible assets
Depreciation and other amortization	11,004,000	9,054,000	7,339,000
Other noninterest expense	190,879,000	154,042,000	124,674,000
Income (loss) before taxes	458,397,000	345,522,000	291,553,000
Income tax expense (benefit)	149,321,000	105,709,000	88,466,000
Net income (loss)	309,076,000	239,813,000	203,087,000
Average total assets	16,437,000,000	15,025,000,000	13,194,000,000
Capital expenditures			1,000,000
Discretionary Portfolio [Member]
Segment Reporting Information [Line Items]
Net interest income	66,303,000	63,030,000	23,347,000
Noninterest income	(76,113,000)	98,770,000	(34,383,000)
Total income	(9,810,000)	161,800,000	(11,036,000)
Provision for credit losses	44,305,000	59,309,000	55,810,000
Amortization of core deposit and other intangible assets
Depreciation and other amortization	2,065,000	2,859,000	3,733,000
Other noninterest expense	31,006,000	32,262,000	26,256,000
Income (loss) before taxes	(87,186,000)	67,370,000	(96,835,000)
Income tax expense (benefit)	(54,071,000)	7,975,000	(58,269,000)
Net income (loss)	(33,115,000)	59,395,000	(38,566,000)
Average total assets	16,583,000,000	14,170,000,000	14,690,000,000
Capital expenditures