Allowance for Credit Losses and Credit Quality - Investment in Covered Loans and Non-Covered Loans by Credit Quality Indicator (Detail) - USD ($) $ in Thousands	Jun. 30, 2016	Dec. 31, 2015
Financing Receivable, Allowance for Credit Losses
Total Acquired Loans, Net of Unearned Income	$ 14,511,188	$ 14,098,211
Legacy Loans
Financing Receivable, Allowance for Credit Losses
Current	11,843,494	11,118,859
Total Acquired Loans, Net of Unearned Income	11,984,849	11,190,520
Legacy Loans | Commercial Loans
Financing Receivable, Allowance for Credit Losses
Total Acquired Loans, Net of Unearned Income	8,784,789	8,133,341
Legacy Loans | Commercial Loans | Pass
Financing Receivable, Allowance for Credit Losses
Credit quality indicator by asset risk classification	8,321,127	7,884,470
Legacy Loans | Commercial Loans | Special Mention
Financing Receivable, Allowance for Credit Losses
Credit quality indicator by asset risk classification	121,048	104,800
Legacy Loans | Commercial Loans | Substandard
Financing Receivable, Allowance for Credit Losses
Credit quality indicator by asset risk classification	331,660	132,593
Legacy Loans | Commercial Loans | Doubtful
Financing Receivable, Allowance for Credit Losses
Credit quality indicator by asset risk classification	10,954	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	695,124	635,560
Total Acquired Loans, Net of Unearned Income	695,209	636,481
Legacy Loans | Commercial Loans | Commercial Real Estate - Construction | Pass
Financing Receivable, Allowance for Credit Losses
Credit quality indicator by asset risk classification	695,025	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	158	160
Legacy Loans | Commercial Loans | Commercial Real Estate - Construction | Substandard
Financing Receivable, Allowance for Credit Losses
Credit quality indicator by asset risk classification	26	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	4,385,949	3,848,584
Total Acquired Loans, Net of Unearned Income	4,402,480	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	4,344,218	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	20,213	21,877
Legacy Loans | Commercial Loans | Commercial Real Estate - Other | Substandard
Financing Receivable, Allowance for Credit Losses
Credit quality indicator by asset risk classification	37,735	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	3,003,153	2,943,409
Total Acquired Loans, Net of Unearned Income	3,027,590	2,952,102
Legacy Loans | Commercial Loans | Commercial and Industrial | Pass
Financing Receivable, Allowance for Credit Losses
Credit quality indicator by asset risk classification	2,935,529	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	31,333	14,826
Legacy Loans | Commercial Loans | Commercial and Industrial | Substandard
Financing Receivable, Allowance for Credit Losses
Credit quality indicator by asset risk classification	57,904	19,888
Legacy Loans | Commercial Loans | Commercial and Industrial | Doubtful
Financing Receivable, Allowance for Credit Losses
Credit quality indicator by asset risk classification	2,824	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	595,689	670,081
Total Acquired Loans, Net of Unearned Income	659,510	677,177
Legacy Loans | Commercial Loans | Energy-Related | Pass
Financing Receivable, Allowance for Credit Losses
Credit quality indicator by asset risk classification	346,355	531,657
Legacy Loans | Commercial Loans | Energy-Related | Special Mention
Financing Receivable, Allowance for Credit Losses
Credit quality indicator by asset risk classification	69,344	67,937
Legacy Loans | Commercial Loans | Energy-Related | Substandard
Financing Receivable, Allowance for Credit Losses
Credit quality indicator by asset risk classification	235,995	74,272
Legacy Loans | Commercial Loans | Energy-Related | Doubtful
Financing Receivable, Allowance for Credit Losses
Credit quality indicator by asset risk classification	7,816	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	3,163,579	3,021,225
30+ Days Past Due	36,481	35,954
Total Acquired Loans, Net of Unearned Income	3,200,060	3,057,179
Legacy Loans | Residential Mortgage | Residential Mortgage
Financing Receivable, Allowance for Credit Losses
Current	778,094	676,347
30+ Days Past Due	16,607	17,676
Total Acquired Loans, Net of Unearned Income	794,701	694,023
Legacy Loans | Consumer - Other | Consumer - Home Equity
Financing Receivable, Allowance for Credit Losses
Current	1,684,473	1,565,596
30+ Days Past Due	10,640	10,047
Total Acquired Loans, Net of Unearned Income	1,695,113	1,575,643
Legacy Loans | Consumer - Other | Consumer - Indirect Automobile
Financing Receivable, Allowance for Credit Losses
Current	177,697	242,328
30+ Days Past Due	4,502	3,886
Total Acquired Loans, Net of Unearned Income	182,199	246,214
Legacy Loans | Consumer - Other | Consumer - Credit Card
Financing Receivable, Allowance for Credit Losses
Current	77,144	76,360
30+ Days Past Due	900	901
Total Acquired Loans, Net of Unearned Income	78,044	77,261
Legacy Loans | Consumer - Other | Consumer - Other
Financing Receivable, Allowance for Credit Losses
Current	446,171	460,594
30+ Days Past Due	3,832	3,444
Total Acquired Loans, Net of Unearned Income	450,003	464,038
Acquired Loans
Financing Receivable, Allowance for Credit Losses
Current	2,759,569	3,137,451
Total Acquired Loans, Net of Unearned Income	2,737,712	3,136,908
Acquired Loans | Commercial Loans
Financing Receivable, Allowance for Credit Losses
Total Acquired Loans, Net of Unearned Income	1,785,055	2,065,514
Acquired Loans | Commercial Loans | Pass
Financing Receivable, Allowance for Credit Losses
Credit quality indicator by asset risk classification	1,739,141	1,975,355
Acquired Loans | Commercial Loans | Special Mention
Financing Receivable, Allowance for Credit Losses
Credit quality indicator by asset risk classification	27,842	36,192
Acquired Loans | Commercial Loans | Substandard
Financing Receivable, Allowance for Credit Losses
Credit quality indicator by asset risk classification	66,265	104,602
Acquired Loans | Commercial Loans | Doubtful
Financing Receivable, Allowance for Credit Losses
Credit quality indicator by asset risk classification	5,151	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	53,344	60,088
Acquired Loans | Commercial Loans | Commercial Real Estate - Construction
Financing Receivable, Allowance for Credit Losses
Current	91,715	107,992
Total Acquired Loans, Net of Unearned Income	107,396	125,426
Acquired Loans | Commercial Loans | Commercial Real Estate - Construction | Pass
Financing Receivable, Allowance for Credit Losses
Credit quality indicator by asset risk classification	91,301	104,064
Acquired Loans | Commercial Loans | Commercial Real Estate - Construction | Special Mention
Financing Receivable, Allowance for Credit Losses
Credit quality indicator by asset risk classification	144	1,681
Acquired Loans | Commercial Loans | Commercial Real Estate - Construction | Substandard
Financing Receivable, Allowance for Credit Losses
Credit quality indicator by asset risk classification	3,964	8,803
Acquired Loans | Commercial Loans | Commercial Real Estate - Construction | Doubtful
Financing Receivable, Allowance for Credit Losses
Credit quality indicator by asset risk classification	744	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	(11,243)	(10,107)
Acquired Loans | Commercial Loans | Commercial Real Estate - Other
Financing Receivable, Allowance for Credit Losses
Current	1,276,868	1,447,441
Total Acquired Loans, Net of Unearned Income	1,266,916	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	1,241,282	1,395,884
Acquired Loans | Commercial Loans | Commercial Real Estate - Other | Special Mention
Financing Receivable, Allowance for Credit Losses
Credit quality indicator by asset risk classification	25,260	26,080
Acquired Loans | Commercial Loans | Commercial Real Estate - Other | Substandard
Financing Receivable, Allowance for Credit Losses
Credit quality indicator by asset risk classification	48,040	79,119
Acquired Loans | Commercial Loans | Commercial Real Estate - Other | Doubtful
Financing Receivable, Allowance for Credit Losses
Credit quality indicator by asset risk classification	3,818	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	51,484	63,295
Acquired Loans | Commercial Loans | Commercial and Industrial
Financing Receivable, Allowance for Credit Losses
Current	414,406	490,255
Total Acquired Loans, Net of Unearned Income	408,219	492,476
Acquired Loans | Commercial Loans | Commercial and Industrial | Pass
Financing Receivable, Allowance for Credit Losses
Credit quality indicator by asset risk classification	404,046	473,241
Acquired Loans | Commercial Loans | Commercial and Industrial | Special Mention
Financing Receivable, Allowance for Credit Losses
Credit quality indicator by asset risk classification	2,438	8,376
Acquired Loans | Commercial Loans | Commercial and Industrial | Substandard
Financing Receivable, Allowance for Credit Losses
Credit quality indicator by asset risk classification	14,213	16,510
Acquired Loans | Commercial Loans | Commercial and Industrial | Doubtful
Financing Receivable, Allowance for Credit Losses
Credit quality indicator by asset risk classification	589	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	13,067	6,900
Acquired Loans | Commercial Loans | Energy-Related
Financing Receivable, Allowance for Credit Losses
Current	2,512	2,221
Total Acquired Loans, Net of Unearned Income	2,524	3,589
Acquired Loans | Commercial Loans | Energy-Related | Pass
Financing Receivable, Allowance for Credit Losses
Credit quality indicator by asset risk classification	2,512	2,166
Acquired Loans | Commercial Loans | Energy-Related | Special Mention
Financing Receivable, Allowance for Credit Losses
Credit quality indicator by asset risk classification	0	55
Acquired Loans | Commercial Loans | Energy-Related | Substandard
Financing Receivable, Allowance for Credit Losses
Credit quality indicator by asset risk classification	48	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	36	0
Acquired Loans | Residential and Consumer Portfolio Segment
Financing Receivable, Allowance for Credit Losses
Current	974,068	1,089,542
30+ Days Past Due	39,491	42,620
Premium (discount)	(60,902)	(60,768)
Total Acquired Loans, Net of Unearned Income	952,657	1,071,394
Acquired Loans | Residential Mortgage | Residential Mortgage
Financing Receivable, Allowance for Credit Losses
Current	464,518	506,103
30+ Days Past Due	23,721	24,752
Premium (discount)	(33,878)	(29,559)
Total Acquired Loans, Net of Unearned Income	454,361	501,296
Acquired Loans | Consumer - Other | Consumer - Home Equity
Financing Receivable, Allowance for Credit Losses
Current	446,532	503,635
30+ Days Past Due	14,462	16,381
Premium (discount)	(26,295)	(29,492)
Total Acquired Loans, Net of Unearned Income	434,699	490,524
Acquired Loans | Consumer - Other | Consumer - Indirect Automobile
Financing Receivable, Allowance for Credit Losses
Current	23	72
30+ Days Past Due	2	12
Premium (discount)	(1)	0
Total Acquired Loans, Net of Unearned Income	24	84
Acquired Loans | Consumer - Other | Consumer - Credit Card
Financing Receivable, Allowance for Credit Losses
Current	488	565
Total Acquired Loans, Net of Unearned Income	508	582
Acquired Loans | Consumer - Other | Consumer - Other
Financing Receivable, Allowance for Credit Losses
Current	62,507	79,167
Total Acquired Loans, Net of Unearned Income	63,065	78,908
Acquired Loans | Consumer - Other | Other
Financing Receivable, Allowance for Credit Losses
Current	62,995	79,732
30+ Days Past Due	1,306	1,475
Premium (discount)	(728)	(1,717)
Total Acquired Loans, Net of Unearned Income	$ 63,573	$ 79,490