Allowance for Credit Losses and Credit Quality - Investment in Covered Loans and Non-Covered Loans by Credit Quality Indicator (Detail) - USD ($) $ in Thousands	Mar. 31, 2016	Dec. 31, 2015
Financing Receivable, Allowance for Credit Losses
Total Acquired Loans, Net of Unearned Income	$ 14,230,752	$ 14,098,211
Legacy Loans
Financing Receivable, Allowance for Credit Losses
Current	11,392,689	11,118,859
Total Acquired Loans, Net of Unearned Income	11,528,697	11,190,520
Legacy Loans | Commercial Loans
Financing Receivable, Allowance for Credit Losses
Total Acquired Loans, Net of Unearned Income	8,427,154	8,133,341
Legacy Loans | Commercial Loans | Pass
Financing Receivable, Allowance for Credit Losses
Credit quality indicator by asset risk classification	7,955,444	7,884,470
Legacy Loans | Commercial Loans | Special Mention
Financing Receivable, Allowance for Credit Losses
Credit quality indicator by asset risk classification	114,684	104,800
Legacy Loans | Commercial Loans | Substandard
Financing Receivable, Allowance for Credit Losses
Credit quality indicator by asset risk classification	350,647	132,593
Legacy Loans | Commercial Loans | Doubtful
Financing Receivable, Allowance for Credit Losses
Credit quality indicator by asset risk classification	6,379	11,478
Legacy Loans | Commercial Loans | Loss
Financing Receivable, Allowance for Credit Losses
Credit quality indicator by asset risk classification	0
Legacy Loans | Commercial Loans | Commercial Real Estate - Construction
Financing Receivable, Allowance for Credit Losses
Current	603,181	635,560
Total Acquired Loans, Net of Unearned Income	613,971	636,481
Legacy Loans | Commercial Loans | Commercial Real Estate - Construction | Pass
Financing Receivable, Allowance for Credit Losses
Credit quality indicator by asset risk classification	613,804	634,889
Legacy Loans | Commercial Loans | Commercial Real Estate - Construction | Special Mention
Financing Receivable, Allowance for Credit Losses
Credit quality indicator by asset risk classification	125	160
Legacy Loans | Commercial Loans | Commercial Real Estate - Construction | Substandard
Financing Receivable, Allowance for Credit Losses
Credit quality indicator by asset risk classification	42	1,432
Legacy Loans | Commercial Loans | Commercial Real Estate - Construction | Doubtful
Financing Receivable, Allowance for Credit Losses
Credit quality indicator by asset risk classification	0	0
Legacy Loans | Commercial Loans | Commercial Real Estate - Construction | Loss
Financing Receivable, Allowance for Credit Losses
Credit quality indicator by asset risk classification	0
Legacy Loans | Commercial Loans | Commercial Real Estate - Other
Financing Receivable, Allowance for Credit Losses
Current	4,132,299	3,848,584
Total Acquired Loans, Net of Unearned Income	4,157,719	3,867,581
Legacy Loans | Commercial Loans | Commercial Real Estate - Other | Pass
Financing Receivable, Allowance for Credit Losses
Credit quality indicator by asset risk classification	4,098,025	3,806,528
Legacy Loans | Commercial Loans | Commercial Real Estate - Other | Special Mention
Financing Receivable, Allowance for Credit Losses
Credit quality indicator by asset risk classification	18,736	21,877
Legacy Loans | Commercial Loans | Commercial Real Estate - Other | Substandard
Financing Receivable, Allowance for Credit Losses
Credit quality indicator by asset risk classification	40,644	37,001
Legacy Loans | Commercial Loans | Commercial Real Estate - Other | Doubtful
Financing Receivable, Allowance for Credit Losses
Credit quality indicator by asset risk classification	314	2,175
Legacy Loans | Commercial Loans | Commercial Real Estate - Other | Loss
Financing Receivable, Allowance for Credit Losses
Credit quality indicator by asset risk classification	0
Legacy Loans | Commercial Loans | Commercial and Industrial
Financing Receivable, Allowance for Credit Losses
Current	2,908,331	2,943,409
Total Acquired Loans, Net of Unearned Income	2,926,686	2,952,102
Legacy Loans | Commercial Loans | Commercial and Industrial | Pass
Financing Receivable, Allowance for Credit Losses
Credit quality indicator by asset risk classification	2,871,108	2,911,396
Legacy Loans | Commercial Loans | Commercial and Industrial | Special Mention
Financing Receivable, Allowance for Credit Losses
Credit quality indicator by asset risk classification	26,940	14,826
Legacy Loans | Commercial Loans | Commercial and Industrial | Substandard
Financing Receivable, Allowance for Credit Losses
Credit quality indicator by asset risk classification	25,857	19,888
Legacy Loans | Commercial Loans | Commercial and Industrial | Doubtful
Financing Receivable, Allowance for Credit Losses
Credit quality indicator by asset risk classification	2,781	5,992
Legacy Loans | Commercial Loans | Commercial and Industrial | Loss
Financing Receivable, Allowance for Credit Losses
Credit quality indicator by asset risk classification	0
Legacy Loans | Commercial Loans | Energy-Related
Financing Receivable, Allowance for Credit Losses
Current	682,627	670,081
Total Acquired Loans, Net of Unearned Income	728,778	677,177
Legacy Loans | Commercial Loans | Energy-Related | Pass
Financing Receivable, Allowance for Credit Losses
Credit quality indicator by asset risk classification	372,507	531,657
Legacy Loans | Commercial Loans | Energy-Related | Special Mention
Financing Receivable, Allowance for Credit Losses
Credit quality indicator by asset risk classification	68,883	67,937
Legacy Loans | Commercial Loans | Energy-Related | Substandard
Financing Receivable, Allowance for Credit Losses
Credit quality indicator by asset risk classification	284,104	74,272
Legacy Loans | Commercial Loans | Energy-Related | Doubtful
Financing Receivable, Allowance for Credit Losses
Credit quality indicator by asset risk classification	3,284	3,311
Legacy Loans | Commercial Loans | Energy-Related | Loss
Financing Receivable, Allowance for Credit Losses
Credit quality indicator by asset risk classification	0
Legacy Loans | Residential and Consumer Portfolio Segment
Financing Receivable, Allowance for Credit Losses
Current	3,066,251	3,021,225
30+ Days Past Due	35,292	35,954
Total Acquired Loans, Net of Unearned Income	3,101,543	3,057,179
Legacy Loans | Residential Mortgage | Residential Mortgage
Financing Receivable, Allowance for Credit Losses
Current	713,420	676,347
30+ Days Past Due	17,201	17,676
Total Acquired Loans, Net of Unearned Income	730,621	694,023
Legacy Loans | Consumer - Other | Consumer - Home Equity
Financing Receivable, Allowance for Credit Losses
Current	1,615,227	1,565,596
30+ Days Past Due	10,585	10,047
Total Acquired Loans, Net of Unearned Income	1,625,812	1,575,643
Legacy Loans | Consumer - Other | Consumer - Indirect Automobile
Financing Receivable, Allowance for Credit Losses
Current	209,814	242,328
30+ Days Past Due	3,327	3,886
Total Acquired Loans, Net of Unearned Income	213,141	246,214
Legacy Loans | Consumer - Other | Consumer - Credit Card
Financing Receivable, Allowance for Credit Losses
Current	75,514	76,360
30+ Days Past Due	733	901
Total Acquired Loans, Net of Unearned Income	76,247	77,261
Legacy Loans | Consumer - Other | Consumer - Other
Financing Receivable, Allowance for Credit Losses
Current	452,276	460,594
30+ Days Past Due	3,446	3,444
Total Acquired Loans, Net of Unearned Income	455,722	464,038
Acquired Loans
Financing Receivable, Allowance for Credit Losses
Current	2,935,609	3,137,451
Total Acquired Loans, Net of Unearned Income	2,922,547	3,136,908
Acquired Loans | Commercial Loans
Financing Receivable, Allowance for Credit Losses
Total Acquired Loans, Net of Unearned Income	1,909,518	2,065,514
Acquired Loans | Commercial Loans | Pass
Financing Receivable, Allowance for Credit Losses
Credit quality indicator by asset risk classification	1,847,083	1,975,355
Acquired Loans | Commercial Loans | Special Mention
Financing Receivable, Allowance for Credit Losses
Credit quality indicator by asset risk classification	29,842	36,192
Acquired Loans | Commercial Loans | Substandard
Financing Receivable, Allowance for Credit Losses
Credit quality indicator by asset risk classification	86,946	104,602
Acquired Loans | Commercial Loans | Doubtful
Financing Receivable, Allowance for Credit Losses
Credit quality indicator by asset risk classification	5,677	9,299
Acquired Loans | Commercial Loans | Loss
Financing Receivable, Allowance for Credit Losses
Credit quality indicator by asset risk classification	0	154
Acquired Loans | Commercial Loans | Discount
Financing Receivable, Allowance for Credit Losses
Credit quality indicator by asset risk classification	60,030	60,088
Acquired Loans | Commercial Loans | Commercial Real Estate - Construction
Financing Receivable, Allowance for Credit Losses
Current	104,280	120,467
Total Acquired Loans, Net of Unearned Income	126,605	125,426
Acquired Loans | Commercial Loans | Commercial Real Estate - Construction | Pass
Financing Receivable, Allowance for Credit Losses
Credit quality indicator by asset risk classification	101,850	116,539
Acquired Loans | Commercial Loans | Commercial Real Estate - Construction | Special Mention
Financing Receivable, Allowance for Credit Losses
Credit quality indicator by asset risk classification	621	1,681
Acquired Loans | Commercial Loans | Commercial Real Estate - Construction | Substandard
Financing Receivable, Allowance for Credit Losses
Credit quality indicator by asset risk classification	8,421	8,803
Acquired Loans | Commercial Loans | Commercial Real Estate - Construction | Doubtful
Financing Receivable, Allowance for Credit Losses
Credit quality indicator by asset risk classification	770	771
Acquired Loans | Commercial Loans | Commercial Real Estate - Construction | Loss
Financing Receivable, Allowance for Credit Losses
Credit quality indicator by asset risk classification	0	0
Acquired Loans | Commercial Loans | Commercial Real Estate - Construction | Discount
Financing Receivable, Allowance for Credit Losses
Credit quality indicator by asset risk classification	(14,943)	2,368
Acquired Loans | Commercial Loans | Commercial Real Estate - Other
Financing Receivable, Allowance for Credit Losses
Current	1,349,646	1,434,966
Total Acquired Loans, Net of Unearned Income	1,332,333	1,444,023
Acquired Loans | Commercial Loans | Commercial Real Estate - Other | Pass
Financing Receivable, Allowance for Credit Losses
Credit quality indicator by asset risk classification	1,312,852	1,383,409
Acquired Loans | Commercial Loans | Commercial Real Estate - Other | Special Mention
Financing Receivable, Allowance for Credit Losses
Credit quality indicator by asset risk classification	22,392	26,080
Acquired Loans | Commercial Loans | Commercial Real Estate - Other | Substandard
Financing Receivable, Allowance for Credit Losses
Credit quality indicator by asset risk classification	63,735	79,119
Acquired Loans | Commercial Loans | Commercial Real Estate - Other | Doubtful
Financing Receivable, Allowance for Credit Losses
Credit quality indicator by asset risk classification	3,748	6,124
Acquired Loans | Commercial Loans | Commercial Real Estate - Other | Loss
Financing Receivable, Allowance for Credit Losses
Credit quality indicator by asset risk classification	0	111
Acquired Loans | Commercial Loans | Commercial Real Estate - Other | Discount
Financing Receivable, Allowance for Credit Losses
Credit quality indicator by asset risk classification	70,394	50,820
Acquired Loans | Commercial Loans | Commercial and Industrial
Financing Receivable, Allowance for Credit Losses
Current	443,954	490,255
Total Acquired Loans, Net of Unearned Income	447,696	492,476
Acquired Loans | Commercial Loans | Commercial and Industrial | Pass
Financing Receivable, Allowance for Credit Losses
Credit quality indicator by asset risk classification	429,631	473,241
Acquired Loans | Commercial Loans | Commercial and Industrial | Special Mention
Financing Receivable, Allowance for Credit Losses
Credit quality indicator by asset risk classification	6,776	8,376
Acquired Loans | Commercial Loans | Commercial and Industrial | Substandard
Financing Receivable, Allowance for Credit Losses
Credit quality indicator by asset risk classification	14,709	16,510
Acquired Loans | Commercial Loans | Commercial and Industrial | Doubtful
Financing Receivable, Allowance for Credit Losses
Credit quality indicator by asset risk classification	1,159	1,206
Acquired Loans | Commercial Loans | Commercial and Industrial | Loss
Financing Receivable, Allowance for Credit Losses
Credit quality indicator by asset risk classification	0	43
Acquired Loans | Commercial Loans | Commercial and Industrial | Discount
Financing Receivable, Allowance for Credit Losses
Credit quality indicator by asset risk classification	4,579	6,900
Acquired Loans | Commercial Loans | Energy-Related
Financing Receivable, Allowance for Credit Losses
Current	2,803	2,221
Total Acquired Loans, Net of Unearned Income	2,884	3,589
Acquired Loans | Commercial Loans | Energy-Related | Pass
Financing Receivable, Allowance for Credit Losses
Credit quality indicator by asset risk classification	2,750	2,166
Acquired Loans | Commercial Loans | Energy-Related | Special Mention
Financing Receivable, Allowance for Credit Losses
Credit quality indicator by asset risk classification	53	55
Acquired Loans | Commercial Loans | Energy-Related | Substandard
Financing Receivable, Allowance for Credit Losses
Credit quality indicator by asset risk classification	81	170
Acquired Loans | Commercial Loans | Energy-Related | Doubtful
Financing Receivable, Allowance for Credit Losses
Credit quality indicator by asset risk classification	0	1,198
Acquired Loans | Commercial Loans | Energy-Related | Loss
Financing Receivable, Allowance for Credit Losses
Credit quality indicator by asset risk classification	0	0
Acquired Loans | Commercial Loans | Energy-Related | Discount
Financing Receivable, Allowance for Credit Losses
Credit quality indicator by asset risk classification	0	0
Acquired Loans | Residential and Consumer Portfolio Segment
Financing Receivable, Allowance for Credit Losses
Current	1,034,926	1,089,542
30+ Days Past Due	38,025	42,620
Premium (discount)	(59,922)	(60,768)
Total Acquired Loans, Net of Unearned Income	1,013,029	1,071,394
Acquired Loans | Residential Mortgage | Residential Mortgage
Financing Receivable, Allowance for Credit Losses
Current	486,566	506,103
30+ Days Past Due	22,430	24,752
Premium (discount)	(31,226)	(29,559)
Total Acquired Loans, Net of Unearned Income	477,770	501,296
Acquired Loans | Consumer - Other | Consumer - Home Equity
Financing Receivable, Allowance for Credit Losses
Current	478,837	503,635
30+ Days Past Due	14,381	16,381
Premium (discount)	(27,516)	(29,492)
Total Acquired Loans, Net of Unearned Income	465,702	490,524
Acquired Loans | Consumer - Other | Consumer - Indirect Automobile
Financing Receivable, Allowance for Credit Losses
Current	66	72
30+ Days Past Due	5	12
Premium (discount)	(33)	0
Total Acquired Loans, Net of Unearned Income	38	84
Acquired Loans | Consumer - Other | Consumer - Credit Card
Financing Receivable, Allowance for Credit Losses
Current	486	565
Total Acquired Loans, Net of Unearned Income	509	582
Acquired Loans | Consumer - Other | Consumer - Other
Financing Receivable, Allowance for Credit Losses
Current	68,971	79,167
Total Acquired Loans, Net of Unearned Income	69,010	78,908
Acquired Loans | Consumer - Other | Other
Financing Receivable, Allowance for Credit Losses
Current	69,457	79,732
30+ Days Past Due	1,209	1,475
Premium (discount)	(1,147)	(1,717)
Total Acquired Loans, Net of Unearned Income	$ 69,519	$ 79,490