Allowance for Credit Losses and Credit Quality - Investment in Covered Loans and Non-Covered Loans by Credit Quality Indicator (Detail) - USD ($) $ in Thousands	Sep. 30, 2016	Dec. 31, 2015
Financing Receivable, Allowance for Credit Losses
Premium (Discount)	$ (20,900)	$ (18,700)
Non-covered loans, net of unearned income	14,722,288	14,098,211
Legacy Loans
Financing Receivable, Allowance for Credit Losses
Non-covered loans, net of unearned income	12,413,370	11,190,520
Legacy Loans | Commercial Loans
Financing Receivable, Allowance for Credit Losses
Non-covered loans, net of unearned income	9,119,234	8,133,341
Legacy Loans | Commercial Loans | Pass
Financing Receivable, Allowance for Credit Losses
Credit quality indicator by asset risk classification	8,623,667	7,884,470
Legacy Loans | Commercial Loans | Special Mention
Financing Receivable, Allowance for Credit Losses
Credit quality indicator by asset risk classification	122,515	104,800
Legacy Loans | Commercial Loans | Substandard
Financing Receivable, Allowance for Credit Losses
Credit quality indicator by asset risk classification	350,307	132,593
Legacy Loans | Commercial Loans | Doubtful
Financing Receivable, Allowance for Credit Losses
Credit quality indicator by asset risk classification	22,745	11,478
Legacy Loans | Commercial Loans | Loss
Financing Receivable, Allowance for Credit Losses
Credit quality indicator by asset risk classification	0	0
Legacy Loans | Commercial Loans | Commercial Real Estate - Construction
Financing Receivable, Allowance for Credit Losses
Non-covered loans, net of unearned income	696,127	636,481
Legacy Loans | Commercial Loans | Commercial Real Estate - Construction | Pass
Financing Receivable, Allowance for Credit Losses
Credit quality indicator by asset risk classification	694,983	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	84	160
Legacy Loans | Commercial Loans | Commercial Real Estate - Construction | Substandard
Financing Receivable, Allowance for Credit Losses
Credit quality indicator by asset risk classification	1,060	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	0
Legacy Loans | Commercial Loans | Commercial Real Estate - Other
Financing Receivable, Allowance for Credit Losses
Non-covered loans, net of unearned income	4,723,356	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	4,654,619	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	22,919	21,877
Legacy Loans | Commercial Loans | Commercial Real Estate - Other | Substandard
Financing Receivable, Allowance for Credit Losses
Credit quality indicator by asset risk classification	45,656	37,001
Legacy Loans | Commercial Loans | Commercial Real Estate - Other | Doubtful
Financing Receivable, Allowance for Credit Losses
Credit quality indicator by asset risk classification	162	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	0
Legacy Loans | Commercial Loans | Commercial and Industrial
Financing Receivable, Allowance for Credit Losses
Non-covered loans, net of unearned income	3,101,472	2,952,102
Legacy Loans | Commercial Loans | Commercial and Industrial | Pass
Financing Receivable, Allowance for Credit Losses
Credit quality indicator by asset risk classification	2,994,661	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	34,249	14,826
Legacy Loans | Commercial Loans | Commercial and Industrial | Substandard
Financing Receivable, Allowance for Credit Losses
Credit quality indicator by asset risk classification	53,951	19,888
Legacy Loans | Commercial Loans | Commercial and Industrial | Doubtful
Financing Receivable, Allowance for Credit Losses
Credit quality indicator by asset risk classification	18,611	5,992
Legacy Loans | Commercial Loans | Commercial and Industrial | Loss
Financing Receivable, Allowance for Credit Losses
Credit quality indicator by asset risk classification	0	0
Legacy Loans | Commercial Loans | Energy-Related
Financing Receivable, Allowance for Credit Losses
Non-covered loans, net of unearned income	598,279	677,177
Legacy Loans | Commercial Loans | Energy-Related | Pass
Financing Receivable, Allowance for Credit Losses
Credit quality indicator by asset risk classification	279,404	531,657
Legacy Loans | Commercial Loans | Energy-Related | Special Mention
Financing Receivable, Allowance for Credit Losses
Credit quality indicator by asset risk classification	65,263	67,937
Legacy Loans | Commercial Loans | Energy-Related | Substandard
Financing Receivable, Allowance for Credit Losses
Credit quality indicator by asset risk classification	249,640	74,272
Legacy Loans | Commercial Loans | Energy-Related | Doubtful
Financing Receivable, Allowance for Credit Losses
Credit quality indicator by asset risk classification	3,972	3,311
Legacy Loans | Commercial Loans | Energy-Related | Loss
Financing Receivable, Allowance for Credit Losses
Credit quality indicator by asset risk classification	0	0
Legacy Loans | Residential and Consumer Portfolio Segment
Financing Receivable, Allowance for Credit Losses
Non-covered loans, net of unearned income	3,294,136	3,057,179
Current	3,246,299	3,021,225
30+ Days Past Due	47,837	35,954
Legacy Loans | Residential Mortgage | Residential Mortgage
Financing Receivable, Allowance for Credit Losses
Non-covered loans, net of unearned income	840,082	694,023
Current	817,154	676,347
30+ Days Past Due	22,928	17,676
Legacy Loans | Consumer | Consumer - Home Equity
Financing Receivable, Allowance for Credit Losses
Non-covered loans, net of unearned income	1,755,295	1,575,643
Current	1,740,001	1,565,596
30+ Days Past Due	15,294	10,047
Legacy Loans | Consumer | Consumer - Indirect Automobile
Financing Receivable, Allowance for Credit Losses
Non-covered loans, net of unearned income	153,904	246,214
Current	150,008	242,328
30+ Days Past Due	3,896	3,886
Legacy Loans | Consumer | Consumer - Credit Card
Financing Receivable, Allowance for Credit Losses
Non-covered loans, net of unearned income	80,452	77,261
Current	79,588	76,360
30+ Days Past Due	864	901
Legacy Loans | Consumer | Consumer - Other
Financing Receivable, Allowance for Credit Losses
Non-covered loans, net of unearned income	464,403	464,038
Current	459,548	460,594
30+ Days Past Due	4,855	3,444
Acquired Loans
Financing Receivable, Allowance for Credit Losses
Non-covered loans, net of unearned income	2,511,129	3,136,908
Acquired Loans | Commercial Loans
Financing Receivable, Allowance for Credit Losses
Premium (Discount)	(44,654)	(60,088)
Non-covered loans, net of unearned income	1,624,619	2,065,514
Acquired Loans | Commercial Loans | Pass
Financing Receivable, Allowance for Credit Losses
Credit quality indicator by asset risk classification	1,571,826	1,975,355
Acquired Loans | Commercial Loans | Special Mention
Financing Receivable, Allowance for Credit Losses
Credit quality indicator by asset risk classification	28,087	36,192
Acquired Loans | Commercial Loans | Substandard
Financing Receivable, Allowance for Credit Losses
Credit quality indicator by asset risk classification	66,551	104,602
Acquired Loans | Commercial Loans | Doubtful
Financing Receivable, Allowance for Credit Losses
Credit quality indicator by asset risk classification	2,786	9,299
Acquired Loans | Commercial Loans | Loss
Financing Receivable, Allowance for Credit Losses
Credit quality indicator by asset risk classification	23	154
Acquired Loans | Commercial Loans | Commercial Real Estate - Construction
Financing Receivable, Allowance for Credit Losses
Premium (Discount)	11,605	10,107
Non-covered loans, net of unearned income	80,795	125,426
Acquired Loans | Commercial Loans | Commercial Real Estate - Construction | Pass
Financing Receivable, Allowance for Credit Losses
Credit quality indicator by asset risk classification	63,139	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	140	1,681
Acquired Loans | Commercial Loans | Commercial Real Estate - Construction | Substandard
Financing Receivable, Allowance for Credit Losses
Credit quality indicator by asset risk classification	5,167	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 - Other
Financing Receivable, Allowance for Credit Losses
Premium (Discount)	(35,184)	(63,295)
Non-covered loans, net of unearned income	1,180,937	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	1,145,942	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	21,731	26,080
Acquired Loans | Commercial Loans | Commercial Real Estate - Other | Substandard
Financing Receivable, Allowance for Credit Losses
Credit quality indicator by asset risk classification	46,941	79,119
Acquired Loans | Commercial Loans | Commercial Real Estate - Other | Doubtful
Financing Receivable, Allowance for Credit Losses
Credit quality indicator by asset risk classification	1,484	6,124
Acquired Loans | Commercial Loans | Commercial Real Estate - Other | Loss
Financing Receivable, Allowance for Credit Losses
Credit quality indicator by asset risk classification	23	111
Acquired Loans | Commercial Loans | Commercial and Industrial
Financing Receivable, Allowance for Credit Losses
Premium (Discount)	(21,068)	(6,900)
Non-covered loans, net of unearned income	361,525	492,476
Acquired Loans | Commercial Loans | Commercial and Industrial | Pass
Financing Receivable, Allowance for Credit Losses
Credit quality indicator by asset risk classification	361,376	473,241
Acquired Loans | Commercial Loans | Commercial and Industrial | Special Mention
Financing Receivable, Allowance for Credit Losses
Credit quality indicator by asset risk classification	6,216	8,376
Acquired Loans | Commercial Loans | Commercial and Industrial | Substandard
Financing Receivable, Allowance for Credit Losses
Credit quality indicator by asset risk classification	14,443	16,510
Acquired Loans | Commercial Loans | Commercial and Industrial | Doubtful
Financing Receivable, Allowance for Credit Losses
Credit quality indicator by asset risk classification	558	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 | Energy-Related
Financing Receivable, Allowance for Credit Losses
Premium (Discount)	(7)	0
Non-covered loans, net of unearned income	1,362	3,589
Acquired Loans | Commercial Loans | Energy-Related | Pass
Financing Receivable, Allowance for Credit Losses
Credit quality indicator by asset risk classification	1,369	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	0	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 | Residential and Consumer Portfolio Segment
Financing Receivable, Allowance for Credit Losses
Premium (Discount)	(57,358)	(60,768)
Non-covered loans, net of unearned income	886,510	1,071,394
Current	908,959	1,089,542
30+ Days Past Due	34,909	42,620
Acquired Loans | Residential Mortgage | Residential Mortgage
Financing Receivable, Allowance for Credit Losses
Premium (Discount)	(32,795)	(29,559)
Non-covered loans, net of unearned income	430,448	501,296
Current	441,411	506,103
30+ Days Past Due	21,832	24,752
Acquired Loans | Consumer | Consumer - Home Equity
Financing Receivable, Allowance for Credit Losses
Premium (Discount)	(24,164)	(29,492)
Non-covered loans, net of unearned income	395,835	490,524
Current	408,541	503,635
30+ Days Past Due	11,458	16,381
Acquired Loans | Consumer | Consumer - Indirect Automobile
Financing Receivable, Allowance for Credit Losses
Premium (Discount)	(1)	0
Non-covered loans, net of unearned income	9	84
Current	9	72
30+ Days Past Due	1	12
Acquired Loans | Consumer | Consumer - Credit Card
Financing Receivable, Allowance for Credit Losses
Non-covered loans, net of unearned income	507	582
Acquired Loans | Consumer | Consumer - Other
Financing Receivable, Allowance for Credit Losses
Premium (Discount)	(398)	(1,717)
Non-covered loans, net of unearned income	60,218	79,490
Current	58,998	79,732
30+ Days Past Due	1,618	1,475
Acquired Loans | Consumer | Consumer - Other
Financing Receivable, Allowance for Credit Losses
Non-covered loans, net of unearned income	$ 59,711	$ 78,908