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, 2015	Dec. 31, 2014		Sep. 30, 2014
Non-covered Acquired Loans [Abstract]
Non-covered loans, net of unearned income	$ 13,863,478	$ 10,996,500
Total covered loans	253,541	444,544
Non-Covered Legacy Loans
Non-covered Acquired Loans [Abstract]
Current	10,710,746	9,603,423
Non-covered loans, net of unearned income	10,779,258	9,668,714
Non-Covered Acquired Loans
Non-covered Acquired Loans [Abstract]
Current	3,073,002	1,326,950
Non-covered loans, net of unearned income	3,084,220	1,327,786
Covered Loans
Non-covered Acquired Loans [Abstract]
Total covered loans	253,541	444,544	[1]	$ 524,189
Commercial Loans | Non-Covered Legacy Loans
Non-covered Legacy Loans
Total	7,815,161	7,002,198
Non-covered Acquired Loans [Abstract]
Non-covered loans, net of unearned income	7,815,161	7,002,198
Commercial Loans | Non-Covered Legacy Loans | Pass
Non-covered Legacy Loans
Credit quality indicator by asset risk classification	7,645,964	6,891,435
Commercial Loans | Non-Covered Legacy Loans | Special Mention
Non-covered Legacy Loans
Credit quality indicator by asset risk classification	59,221	57,417
Commercial Loans | Non-Covered Legacy Loans | Substandard
Non-covered Legacy Loans
Credit quality indicator by asset risk classification	108,723	51,251
Commercial Loans | Non-Covered Legacy Loans | Doubtful
Non-covered Legacy Loans
Credit quality indicator by asset risk classification	1,033	2,095
Commercial Loans | Non-Covered Legacy Loans | Loss
Non-covered Legacy Loans
Credit quality indicator by asset risk classification	220
Commercial Loans | Non-Covered Legacy Loans | Commercial Real Estate - Construction
Non-covered Legacy Loans
Total	559,208	484,239
Non-covered Acquired Loans [Abstract]
Current	559,028	483,663
Non-covered loans, net of unearned income	559,208	484,239
Commercial Loans | Non-Covered Legacy Loans | Commercial Real Estate - Construction | Pass
Non-covered Legacy Loans
Credit quality indicator by asset risk classification	557,603	483,930
Commercial Loans | Non-Covered Legacy Loans | Commercial Real Estate - Construction | Special Mention
Non-covered Legacy Loans
Credit quality indicator by asset risk classification	100	240
Commercial Loans | Non-Covered Legacy Loans | Commercial Real Estate - Construction | Substandard
Non-covered Legacy Loans
Credit quality indicator by asset risk classification	1,503	69
Commercial Loans | Non-Covered Legacy Loans | Commercial Real Estate - Construction | Doubtful
Non-covered Legacy Loans
Credit quality indicator by asset risk classification	0	0
Commercial Loans | Non-Covered Legacy Loans | Commercial Real Estate - Construction | Loss
Non-covered Legacy Loans
Credit quality indicator by asset risk classification	2
Commercial Loans | Non-Covered Legacy Loans | Commercial Real Estate - Other
Non-covered Legacy Loans
Total	3,762,515	3,192,572
Non-covered Acquired Loans [Abstract]
Current	3,744,986	3,173,766
Non-covered loans, net of unearned income	3,762,515	3,192,572
Commercial Loans | Non-Covered Legacy Loans | Commercial Real Estate - Other | Pass
Non-covered Legacy Loans
Credit quality indicator by asset risk classification	3,699,714	3,120,370
Commercial Loans | Non-Covered Legacy Loans | Commercial Real Estate - Other | Special Mention
Non-covered Legacy Loans
Credit quality indicator by asset risk classification	24,473	49,847
Commercial Loans | Non-Covered Legacy Loans | Commercial Real Estate - Other | Substandard
Non-covered Legacy Loans
Credit quality indicator by asset risk classification	37,874	22,193
Commercial Loans | Non-Covered Legacy Loans | Commercial Real Estate - Other | Doubtful
Non-covered Legacy Loans
Credit quality indicator by asset risk classification	454	162
Commercial Loans | Non-Covered Legacy Loans | Commercial Real Estate - Other | Loss
Non-covered Legacy Loans
Credit quality indicator by asset risk classification	0
Commercial Loans | Non-Covered Legacy Loans | Commercial and Industrial
Non-covered Legacy Loans
Total	2,779,503	2,452,521
Non-covered Acquired Loans [Abstract]
Current	2,766,605	2,445,847
Non-covered loans, net of unearned income	2,779,503	2,452,521
Commercial Loans | Non-Covered Legacy Loans | Commercial and Industrial | Pass
Non-covered Legacy Loans
Credit quality indicator by asset risk classification	2,732,122	2,414,293
Commercial Loans | Non-Covered Legacy Loans | Commercial and Industrial | Special Mention
Non-covered Legacy Loans
Credit quality indicator by asset risk classification	18,821	7,330
Commercial Loans | Non-Covered Legacy Loans | Commercial and Industrial | Substandard
Non-covered Legacy Loans
Credit quality indicator by asset risk classification	27,763	28,965
Commercial Loans | Non-Covered Legacy Loans | Commercial and Industrial | Doubtful
Non-covered Legacy Loans
Credit quality indicator by asset risk classification	579	1,933
Commercial Loans | Non-Covered Legacy Loans | Commercial and Industrial | Loss
Non-covered Legacy Loans
Credit quality indicator by asset risk classification	218
Commercial Loans | Non-Covered Legacy Loans | Energy-Related
Non-covered Legacy Loans
Total	713,935	872,866
Non-covered Acquired Loans [Abstract]
Current	710,457	872,839
Non-covered loans, net of unearned income	713,935	872,866
Commercial Loans | Non-Covered Legacy Loans | Energy-Related | Pass
Non-covered Legacy Loans
Credit quality indicator by asset risk classification	656,525	872,842
Commercial Loans | Non-Covered Legacy Loans | Energy-Related | Special Mention
Non-covered Legacy Loans
Credit quality indicator by asset risk classification	15,827	0
Commercial Loans | Non-Covered Legacy Loans | Energy-Related | Substandard
Non-covered Legacy Loans
Credit quality indicator by asset risk classification	41,583	24
Commercial Loans | Non-Covered Legacy Loans | Energy-Related | Doubtful
Non-covered Legacy Loans
Credit quality indicator by asset risk classification	0	0
Commercial Loans | Non-Covered Legacy Loans | Energy-Related | Loss
Non-covered Legacy Loans
Credit quality indicator by asset risk classification	0
Commercial Loans | Non-Covered Acquired Loans
Non-covered Acquired Loans [Abstract]
Non-covered loans, net of unearned income	2,132,008	591,498
Commercial Loans | Non-Covered Acquired Loans | Pass
Non-covered Legacy Loans
Credit quality indicator by asset risk classification	2,051,188	561,512
Commercial Loans | Non-Covered Acquired Loans | Special Mention
Non-covered Legacy Loans
Credit quality indicator by asset risk classification	29,089	16,661
Commercial Loans | Non-Covered Acquired Loans | Substandard
Non-covered Legacy Loans
Credit quality indicator by asset risk classification	91,919	58,581
Commercial Loans | Non-Covered Acquired Loans | Doubtful
Non-covered Legacy Loans
Credit quality indicator by asset risk classification	5,139	257
Commercial Loans | Non-Covered Acquired Loans | Loss
Non-covered Legacy Loans
Credit quality indicator by asset risk classification	539
Commercial Loans | Non-Covered Acquired Loans | Discount
Non-covered Acquired Loans [Abstract]
Premium (discount)	(45,866)	(45,513)
Commercial Loans | Non-Covered Acquired Loans | Commercial Real Estate - Construction
Non-covered Acquired Loans [Abstract]
Current	118,084	26,667
Non-covered loans, net of unearned income	123,761	29,578
Commercial Loans | Non-Covered Acquired Loans | Commercial Real Estate - Construction | Pass
Non-covered Legacy Loans
Credit quality indicator by asset risk classification	114,684	24,118
Commercial Loans | Non-Covered Acquired Loans | Commercial Real Estate - Construction | Special Mention
Non-covered Legacy Loans
Credit quality indicator by asset risk classification	1,226	2,006
Commercial Loans | Non-Covered Acquired Loans | Commercial Real Estate - Construction | Substandard
Non-covered Legacy Loans
Credit quality indicator by asset risk classification	8,538	4,624
Commercial Loans | Non-Covered Acquired Loans | Commercial Real Estate - Construction | Doubtful
Non-covered Legacy Loans
Credit quality indicator by asset risk classification	861	0
Commercial Loans | Non-Covered Acquired Loans | Commercial Real Estate - Construction | Loss
Non-covered Legacy Loans
Credit quality indicator by asset risk classification	0
Commercial Loans | Non-Covered Acquired Loans | Commercial Real Estate - Construction | Discount
Non-covered Acquired Loans [Abstract]
Premium (discount)	(1,548)	(1,170)
Commercial Loans | Non-Covered Acquired Loans | Commercial Real Estate - Other
Non-covered Acquired Loans [Abstract]
Current	1,487,123	473,644
Non-covered loans, net of unearned income	1,490,677	466,264
Commercial Loans | Non-Covered Acquired Loans | Commercial Real Estate - Other | Pass
Non-covered Legacy Loans
Credit quality indicator by asset risk classification	1,434,860	443,450
Commercial Loans | Non-Covered Acquired Loans | Commercial Real Estate - Other | Special Mention
Non-covered Legacy Loans
Credit quality indicator by asset risk classification	23,421	12,794
Commercial Loans | Non-Covered Acquired Loans | Commercial Real Estate - Other | Substandard
Non-covered Legacy Loans
Credit quality indicator by asset risk classification	69,084	49,139
Commercial Loans | Non-Covered Acquired Loans | Commercial Real Estate - Other | Doubtful
Non-covered Legacy Loans
Credit quality indicator by asset risk classification	3,256	0
Commercial Loans | Non-Covered Acquired Loans | Commercial Real Estate - Other | Loss
Non-covered Legacy Loans
Credit quality indicator by asset risk classification	0
Commercial Loans | Non-Covered Acquired Loans | Commercial Real Estate - Other | Discount
Non-covered Acquired Loans [Abstract]
Premium (discount)	(39,944)	(39,119)
Commercial Loans | Non-Covered Acquired Loans | Commercial and Industrial
Non-covered Acquired Loans [Abstract]
Current	508,047	89,338
Non-covered loans, net of unearned income	512,049	87,914
Commercial Loans | Non-Covered Acquired Loans | Commercial and Industrial | Pass
Non-covered Legacy Loans
Credit quality indicator by asset risk classification	498,286	86,213
Commercial Loans | Non-Covered Acquired Loans | Commercial and Industrial | Special Mention
Non-covered Legacy Loans
Credit quality indicator by asset risk classification	3,671	1,861
Commercial Loans | Non-Covered Acquired Loans | Commercial and Industrial | Substandard
Non-covered Legacy Loans
Credit quality indicator by asset risk classification	12,905	4,807
Commercial Loans | Non-Covered Acquired Loans | Commercial and Industrial | Doubtful
Non-covered Legacy Loans
Credit quality indicator by asset risk classification	1,022	257
Commercial Loans | Non-Covered Acquired Loans | Commercial and Industrial | Loss
Non-covered Legacy Loans
Credit quality indicator by asset risk classification	539
Commercial Loans | Non-Covered Acquired Loans | Commercial and Industrial | Discount
Non-covered Acquired Loans [Abstract]
Premium (discount)	(4,374)	(5,224)
Commercial Loans | Non-Covered Acquired Loans | Energy-Related
Non-covered Acquired Loans [Abstract]
Current	3,652	7,731
Non-covered loans, net of unearned income	5,521	7,742
Commercial Loans | Non-Covered Acquired Loans | Energy-Related | Pass
Non-covered Legacy Loans
Credit quality indicator by asset risk classification	3,358	7,731
Commercial Loans | Non-Covered Acquired Loans | Energy-Related | Special Mention
Non-covered Legacy Loans
Credit quality indicator by asset risk classification	771	0
Commercial Loans | Non-Covered Acquired Loans | Energy-Related | Substandard
Non-covered Legacy Loans
Credit quality indicator by asset risk classification	1,392	11
Commercial Loans | Non-Covered Acquired Loans | Energy-Related | Doubtful
Non-covered Legacy Loans
Credit quality indicator by asset risk classification	0	0
Commercial Loans | Non-Covered Acquired Loans | Energy-Related | Loss
Non-covered Legacy Loans
Credit quality indicator by asset risk classification	0
Commercial Loans | Non-Covered Acquired Loans | Energy-Related | Discount
Non-covered Acquired Loans [Abstract]
Premium (discount)	0	0
Commercial Loans | Covered Loans
Non-covered Acquired Loans [Abstract]
Total covered loans	68,899	236,900
Discount	13,890	16,514
Total covered loans	55,009	220,386	[1]
Commercial Loans | Covered Loans | Pass
Non-covered Legacy Loans
Credit quality indicator by asset risk classification	31,570	145,620
Commercial Loans | Covered Loans | Special Mention
Non-covered Legacy Loans
Credit quality indicator by asset risk classification	3,552	22,745
Commercial Loans | Covered Loans | Substandard
Non-covered Legacy Loans
Credit quality indicator by asset risk classification	29,574	68,535
Commercial Loans | Covered Loans | Doubtful
Non-covered Legacy Loans
Credit quality indicator by asset risk classification	4,203	0
Commercial Loans | Covered Loans | Commercial Real Estate - Construction
Non-covered Acquired Loans [Abstract]
Total covered loans	2,047	44,667
Commercial Loans | Covered Loans | Commercial Real Estate - Construction | Pass
Non-covered Legacy Loans
Credit quality indicator by asset risk classification	113	34,731
Commercial Loans | Covered Loans | Commercial Real Estate - Construction | Special Mention
Non-covered Legacy Loans
Credit quality indicator by asset risk classification	820	1,928
Commercial Loans | Covered Loans | Commercial Real Estate - Construction | Substandard
Non-covered Legacy Loans
Credit quality indicator by asset risk classification	23	8,008
Commercial Loans | Covered Loans | Commercial Real Estate - Construction | Doubtful
Non-covered Legacy Loans
Credit quality indicator by asset risk classification	1,091	0
Commercial Loans | Covered Loans | Commercial Real Estate - Other
Non-covered Acquired Loans [Abstract]
Total covered loans	51,201	159,183
Commercial Loans | Covered Loans | Commercial Real Estate - Other | Pass
Non-covered Legacy Loans
Credit quality indicator by asset risk classification	25,820	87,509
Commercial Loans | Covered Loans | Commercial Real Estate - Other | Special Mention
Non-covered Legacy Loans
Credit quality indicator by asset risk classification	1,922	20,422
Commercial Loans | Covered Loans | Commercial Real Estate - Other | Substandard
Non-covered Legacy Loans
Credit quality indicator by asset risk classification	22,705	51,252
Commercial Loans | Covered Loans | Commercial Real Estate - Other | Doubtful
Non-covered Legacy Loans
Credit quality indicator by asset risk classification	754	0
Commercial Loans | Covered Loans | Commercial and Industrial
Non-covered Acquired Loans [Abstract]
Total covered loans	15,651	33,050
Total covered loans	11,419	31,260	[1]	34,234
Commercial Loans | Covered Loans | Commercial and Industrial | Pass
Non-covered Legacy Loans
Credit quality indicator by asset risk classification	5,637	23,380
Commercial Loans | Covered Loans | Commercial and Industrial | Special Mention
Non-covered Legacy Loans
Credit quality indicator by asset risk classification	810	395
Commercial Loans | Covered Loans | Commercial and Industrial | Substandard
Non-covered Legacy Loans
Credit quality indicator by asset risk classification	6,846	9,275
Commercial Loans | Covered Loans | Commercial and Industrial | Doubtful
Non-covered Legacy Loans
Credit quality indicator by asset risk classification	2,358	0
Commercial Loans | Covered Loans | Energy-Related
Non-covered Acquired Loans [Abstract]
Total covered loans	0	0	[1]
Residential and Consumer Portfolio Segment | Non-Covered Legacy Loans
Non-covered Legacy Loans
Current	2,929,670	2,627,308
30+ Days Past Due	34,427	39,208
Total	2,964,097	2,666,516
Residential and Consumer Portfolio Segment | Non-Covered Acquired Loans
Non-covered Acquired Loans [Abstract]
Current	956,096	729,570
30+ Days Past Due	15,161	25,225
Premium (discount)	(19,045)	(18,507)
Non-covered loans, net of unearned income	952,212	736,288
Residential and Consumer Portfolio Segment | Covered Loans
Non-covered Acquired Loans [Abstract]
Current	215,334	241,057
30+ Days Past Due	30,357	38,652
Premium (discount)	(47,159)	(55,551)
Total covered loans	198,532	224,158
Residential Mortgage | Non-Covered Legacy Loans
Non-covered Acquired Loans [Abstract]
Non-covered loans, net of unearned income	660,543	527,694
Residential Mortgage | Non-Covered Legacy Loans | Residential Mortgage
Non-covered Legacy Loans
Current	640,927	508,789
30+ Days Past Due	19,616	18,905
Total	660,543	527,694
Non-covered Acquired Loans [Abstract]
Current	640,927	508,789
Non-covered loans, net of unearned income	660,543	527,694
Residential Mortgage | Non-Covered Acquired Loans
Non-covered Acquired Loans [Abstract]
Non-covered loans, net of unearned income	409,472	424,579
Residential Mortgage | Non-Covered Acquired Loans | Residential Mortgage
Non-covered Acquired Loans [Abstract]
Current	407,673	418,552
30+ Days Past Due	6,401	11,858
Premium (discount)	(4,602)	(5,831)
Non-covered loans, net of unearned income	409,472	424,579
Residential Mortgage | Covered Loans
Non-covered Acquired Loans [Abstract]
Total covered loans	119,926	128,024	[1]	130,976
Residential Mortgage | Covered Loans | Residential Mortgage
Non-covered Acquired Loans [Abstract]
Current	128,804	140,628
30+ Days Past Due	20,672	22,058
Premium (discount)	(29,550)	(34,662)
Total covered loans	119,926	128,024
Consumer and Other Loans | Non-Covered Legacy Loans
Non-covered Acquired Loans [Abstract]
Non-covered loans, net of unearned income	2,303,554	2,138,822
Consumer and Other Loans | Non-Covered Legacy Loans | Consumer - Home Equity
Non-covered Legacy Loans
Current	1,480,368	1,278,865
30+ Days Past Due	8,428	12,111
Total	1,488,796	1,290,976
Non-covered Acquired Loans [Abstract]
Current	1,480,368	1,278,865
Non-covered loans, net of unearned income	1,488,796	1,290,976
Consumer and Other Loans | Non-Covered Legacy Loans | Consumer - Indirect Automobile
Non-covered Legacy Loans
Current	277,996	392,504
30+ Days Past Due	3,526	4,262
Total	281,522	396,766
Non-covered Acquired Loans [Abstract]
Current	277,996	392,504
Non-covered loans, net of unearned income	281,522	396,766
Consumer and Other Loans | Non-Covered Legacy Loans | Consumer - Credit Card
Non-covered Legacy Loans
Current	76,018	71,297
30+ Days Past Due	698	1,448
Total	76,716	72,745
Non-covered Acquired Loans [Abstract]
Current	76,018	71,297
Non-covered loans, net of unearned income	76,716	72,745
Consumer and Other Loans | Non-Covered Legacy Loans | Consumer - Other
Non-covered Legacy Loans
Current	454,361	375,853
30+ Days Past Due	2,159	2,482
Total	456,520	378,335
Non-covered Acquired Loans [Abstract]
Current	454,361	375,853
Non-covered loans, net of unearned income	456,520	378,335
Consumer and Other Loans | Non-Covered Acquired Loans
Non-covered Acquired Loans [Abstract]
Non-covered loans, net of unearned income	542,740	311,709
Consumer and Other Loans | Non-Covered Acquired Loans | Consumer - Home Equity
Non-covered Acquired Loans [Abstract]
Current	452,928	216,310
30+ Days Past Due	7,381	9,808
Premium (discount)	(10,644)	(8,419)
Non-covered loans, net of unearned income	449,665	217,699
Consumer and Other Loans | Non-Covered Acquired Loans | Consumer - Indirect Automobile
Non-covered Acquired Loans [Abstract]
Current	142	393
30+ Days Past Due	24	39
Premium (discount)	(39)	(40)
Non-covered loans, net of unearned income	127	392
Consumer and Other Loans | Non-Covered Acquired Loans | Consumer - Other
Non-covered Acquired Loans [Abstract]
Current	95,353	94,315
30+ Days Past Due	1,355	3,520
Premium (discount)	(3,760)	(4,217)
Non-covered loans, net of unearned income	92,948	93,618
Consumer and Other Loans | Covered Loans
Non-covered Acquired Loans [Abstract]
Total covered loans	78,606	96,134	[1]	$ 111,823
Consumer and Other Loans | Covered Loans | Consumer - Home Equity
Non-covered Acquired Loans [Abstract]
Current	85,772	99,478
30+ Days Past Due	9,643	16,542
Premium (discount)	(18,189)	(23,590)
Total covered loans	77,226	92,430	[1]
Consumer and Other Loans | Covered Loans | Consumer - Indirect Automobile
Non-covered Acquired Loans [Abstract]
Total covered loans	0	0	[1]
Consumer and Other Loans | Covered Loans | Consumer - Credit Card
Non-covered Acquired Loans [Abstract]
Current	542	614
30+ Days Past Due	27	34
Premium (discount)	0	0
Total covered loans	569	648
Consumer and Other Loans | Covered Loans | Consumer - Other
Non-covered Acquired Loans [Abstract]
Current	216	337
30+ Days Past Due	15	18
Premium (discount)	580	2,701
Total covered loans	$ 811	$ 3,056

[1] Included as covered loans at December 31, 2014 is $174.7 million of assets whose reimbursable loss periods ended as of January 1, 2015.