Allowance for credit losses - Impaired Loans and Leases (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Financing Receivable, Impaired [Line Items]
Recorded Investment	$ 1,072,026	$ 1,123,315
Unpaid Principal Balance	1,354,133	1,440,333
Related Allowance	132,682	168,001
Commercial, Financial, Leasing, etc [Member]
Financing Receivable, Impaired [Line Items]
Recorded Investment	159,913	171,642
Unpaid Principal Balance	191,733	206,288
Related Allowance	33,829	48,674
Commercial Real Estate [Member]
Financing Receivable, Impaired [Line Items]
Recorded Investment	199,922	174,522
Unpaid Principal Balance	244,183	219,574
Related Allowance	23,641	17,651
Residential Builder and Developer [Member]
Financing Receivable, Impaired [Line Items]
Recorded Investment	189,613	292,449
Unpaid Principal Balance	321,656	458,146
Related Allowance	25,661	52,562
Other Commercial Construction [Member]
Financing Receivable, Impaired [Line Items]
Recorded Investment	96,562	106,886
Unpaid Principal Balance	100,401	114,501
Related Allowance	6,836	3,836
Residential [Member]
Financing Receivable, Impaired [Line Items]
Recorded Investment	189,793	121,568
Unpaid Principal Balance	218,267	145,123
Related Allowance	3,521	4,420
Residential Alt-A [Member]
Financing Receivable, Impaired [Line Items]
Recorded Investment	160,245	184,752
Unpaid Principal Balance	200,708	223,920
Related Allowance	17,000	25,000
Home equity lines and loans [Member]
Financing Receivable, Impaired [Line Items]
Recorded Investment	12,360	9,385
Unpaid Principal Balance	13,567	10,670
Related Allowance	2,254	2,306
Automobile [Member]
Financing Receivable, Impaired [Line Items]
Recorded Investment	49,210	53,710
Unpaid Principal Balance	49,210	53,710
Related Allowance	14,273	11,468
Other [Member]
Financing Receivable, Impaired [Line Items]
Recorded Investment	14,408	8,401
Unpaid Principal Balance	14,408	8,401
Related Allowance	5,667	2,084
Impaired financing receivable with related allowance [Member]
Financing Receivable, Impaired [Line Items]
Recorded Investment	745,994	724,725
Unpaid Principal Balance	927,411	934,731
Related Allowance	132,682	168,001
Impaired financing receivable with related allowance [Member] | Commercial, Financial, Leasing, etc [Member]
Financing Receivable, Impaired [Line Items]
Recorded Investment	127,282	118,538
Unpaid Principal Balance	149,534	145,510
Related Allowance	33,829	48,674
Impaired financing receivable with related allowance [Member] | Commercial Real Estate [Member]
Financing Receivable, Impaired [Line Items]
Recorded Investment	121,542	102,886
Unpaid Principal Balance	143,846	128,456
Related Allowance	23,641	17,651
Impaired financing receivable with related allowance [Member] | Residential Builder and Developer [Member]
Financing Receivable, Impaired [Line Items]
Recorded Investment	115,306	159,293
Unpaid Principal Balance	216,218	280,869
Related Allowance	25,661	52,562
Impaired financing receivable with related allowance [Member] | Other Commercial Construction [Member]
Financing Receivable, Impaired [Line Items]
Recorded Investment	73,544	20,234
Unpaid Principal Balance	76,869	24,639
Related Allowance	6,836	3,836
Impaired financing receivable with related allowance [Member] | Residential [Member]
Financing Receivable, Impaired [Line Items]
Recorded Investment	103,451	101,882
Unpaid Principal Balance	121,819	119,498
Related Allowance	3,521	4,420
Impaired financing receivable with related allowance [Member] | Residential Alt-A [Member]
Financing Receivable, Impaired [Line Items]
Recorded Investment	128,891	150,396
Unpaid Principal Balance	141,940	162,978
Related Allowance	17,000	25,000
Impaired financing receivable with related allowance [Member] | Home equity lines and loans [Member]
Financing Receivable, Impaired [Line Items]
Recorded Investment	12,360	9,385
Unpaid Principal Balance	13,567	10,670
Related Allowance	2,254	2,306
Impaired financing receivable with related allowance [Member] | Automobile [Member]
Financing Receivable, Impaired [Line Items]
Recorded Investment	49,210	53,710
Unpaid Principal Balance	49,210	53,710
Related Allowance	14,273	11,468
Impaired financing receivable with related allowance [Member] | Other [Member]
Financing Receivable, Impaired [Line Items]
Recorded Investment	14,408	8,401
Unpaid Principal Balance	14,408	8,401
Related Allowance	5,667	2,084
Impaired financing receivable with no related allowance [Member]
Financing Receivable, Impaired [Line Items]
Recorded Investment	326,032	398,590
Unpaid Principal Balance	426,722	505,602
Impaired financing receivable with no related allowance [Member] | Commercial, Financial, Leasing, etc [Member]
Financing Receivable, Impaired [Line Items]
Recorded Investment	32,631	53,104
Unpaid Principal Balance	42,199	60,778
Impaired financing receivable with no related allowance [Member] | Commercial Real Estate [Member]
Financing Receivable, Impaired [Line Items]
Recorded Investment	78,380	71,636
Unpaid Principal Balance	100,337	91,118
Impaired financing receivable with no related allowance [Member] | Residential Builder and Developer [Member]
Financing Receivable, Impaired [Line Items]
Recorded Investment	74,307	133,156
Unpaid Principal Balance	105,438	177,277
Impaired financing receivable with no related allowance [Member] | Other Commercial Construction [Member]
Financing Receivable, Impaired [Line Items]
Recorded Investment	23,018	86,652
Unpaid Principal Balance	23,532	89,862
Impaired financing receivable with no related allowance [Member] | Residential [Member]
Financing Receivable, Impaired [Line Items]
Recorded Investment	86,342	19,686
Unpaid Principal Balance	96,448	25,625
Impaired financing receivable with no related allowance [Member] | Residential Alt-A [Member]
Financing Receivable, Impaired [Line Items]
Recorded Investment	31,354	34,356
Unpaid Principal Balance	$ 58,768	$ 60,942