Allowance for Credit Losses and Credit Quality - Investment in Covered Loans and Non-Covered Loans by Credit Quality Indicator (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended	12 Months Ended
	Mar. 31, 2015	Dec. 31, 2014	Mar. 31, 2014
Financing Receivable, Allowance for Credit Losses [Line Items]
Total	$ 14,812
Total	12,622,341	10,996,500
Balance at end of period	251,120	444,544
Internally Assigned Grade Loss [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Total		591,498
Internally Assigned Grade Loss [Member] | Commercial Real Estate Construction [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Total		29,578
Internally Assigned Grade Loss [Member] | Commercial Real Estate - Other [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Total		468,371
Internally Assigned Grade Loss [Member] | Commercial Business [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Total		93,549
Covered Loans [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Balance at end of period	251,120	444,544	664,295
Covered Loans [Member] | Consumer Loans [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Credit risk by payment status, Current	226,981	241,057
30+ Days Past Due	33,035	38,652
Discount	(46,536)	(55,551)
Balance at end of period	213,480	224,158
Covered Loans [Member] | Residential Prime [Member] | Mortgage [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Credit risk by payment status, Current	133,639	140,628
30+ Days Past Due	20,976	22,058
Discount	(22,848)	(34,662)
Balance at end of period	131,767	128,024
Covered Loans [Member] | Home Equity [Member] | Consumer Loans [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Credit risk by payment status, Current	92,599	99,478
30+ Days Past Due	12,017	16,542
Discount	(23,558)	(23,590)
Balance at end of period	81,058	92,430
Covered Loans [Member] | Credit Card [Member] | Consumer Loans [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Credit risk by payment status, Current	522	614
30+ Days Past Due	25	34
Balance at end of period	547	648
Covered Loans [Member] | Consumer - Other [Member] | Consumer Loans [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Credit risk by payment status, Current	221	337
30+ Days Past Due	17	18
Discount	(130)	2,701
Balance at end of period	108	3,056
Covered Loans [Member] | Commercial Business [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Balance at end of period	12,132		41,695
Covered Loans [Member] | Internally Assigned Grade Loss [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Discount	(23,895)	(16,514)
Total	37,640	220,386
Total	61,535	236,900
Covered Loans [Member] | Internally Assigned Grade Loss [Member] | Commercial Real Estate Construction [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Total	1,489	44,667
Covered Loans [Member] | Internally Assigned Grade Loss [Member] | Commercial Real Estate - Other [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Total	46,444	159,183
Covered Loans [Member] | Internally Assigned Grade Loss [Member] | Commercial Business [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Total	13,602	33,050
Covered Loans [Member] | Pass [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Credit quality indicator by asset risk classification	27,559	145,620
Covered Loans [Member] | Pass [Member] | Commercial Real Estate Construction [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Credit quality indicator by asset risk classification	602	34,731
Covered Loans [Member] | Pass [Member] | Commercial Real Estate - Other [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Credit quality indicator by asset risk classification	20,203	87,509
Covered Loans [Member] | Pass [Member] | Commercial Business [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Credit quality indicator by asset risk classification	6,754	23,380
Covered Loans [Member] | Special Mention [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Credit quality indicator by asset risk classification	8,198	22,745
Covered Loans [Member] | Special Mention [Member] | Commercial Real Estate Construction [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Credit quality indicator by asset risk classification	865	1,928
Covered Loans [Member] | Special Mention [Member] | Commercial Real Estate - Other [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Credit quality indicator by asset risk classification	6,132	20,422
Covered Loans [Member] | Special Mention [Member] | Commercial Business [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Credit quality indicator by asset risk classification	1,201	395
Covered Loans [Member] | Sub-standard [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Credit quality indicator by asset risk classification	25,778	68,535
Covered Loans [Member] | Sub-standard [Member] | Commercial Real Estate Construction [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Credit quality indicator by asset risk classification	22	8,008
Covered Loans [Member] | Sub-standard [Member] | Commercial Real Estate - Other [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Credit quality indicator by asset risk classification	20,109	51,252
Covered Loans [Member] | Sub-standard [Member] | Commercial Business [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Credit quality indicator by asset risk classification	5,647	9,275
Non-Covered Legacy Loans [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Discount	(9,849,416)	(9,668,714)
Current	2,699,909	2,627,308
30+ Days Past Due	38,643	39,208
Total	2,738,552	2,666,516
Credit risk by payment status, Current	9,771,508	9,603,423
Total	9,849,416	9,668,714
Non-Covered Legacy Loans [Member] | Residential Prime [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Current	423,428	392,900
30+ Days Past Due	13,974	15,310
Total	437,402	408,210
Non-Covered Legacy Loans [Member] | Home Equity [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Current	1,322,849	1,278,865
30+ Days Past Due	12,541	12,111
Total	1,335,390	1,290,976
Non-Covered Legacy Loans [Member] | Credit Card [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Current	70,603	71,297
30+ Days Past Due	1,561	1,448
Total	72,164	72,745
Non-Covered Legacy Loans [Member] | Consumer - Other [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Current	407,460	375,853
30+ Days Past Due	2,646	2,482
Total	410,106	378,335
Non-Covered Legacy Loans [Member] | Residential Subprime [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Current	112,431	115,889
30+ Days Past Due	3,982	3,595
Total	116,413	119,484
Non-Covered Legacy Loans [Member] | Indirect Automobile [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Current	363,138	392,504
30+ Days Past Due	3,939	4,262
Total	367,077	396,766
Non-Covered Legacy Loans [Member] | Internally Assigned Grade Loss [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Total	7,110,864	7,002,198
Non-Covered Legacy Loans [Member] | Internally Assigned Grade Loss [Member] | Commercial Real Estate Construction [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Total	521,985	484,239
Non-Covered Legacy Loans [Member] | Internally Assigned Grade Loss [Member] | Commercial Real Estate - Other [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Total	3,310,613	3,233,819
Non-Covered Legacy Loans [Member] | Internally Assigned Grade Loss [Member] | Commercial Business [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Total	3,278,266	3,284,140
Non-Covered Legacy Loans [Member] | Pass [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Credit quality indicator by asset risk classification	6,970,772	6,891,435
Non-Covered Legacy Loans [Member] | Pass [Member] | Commercial Real Estate Construction [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Credit quality indicator by asset risk classification	520,132	483,930
Non-Covered Legacy Loans [Member] | Pass [Member] | Commercial Real Estate - Other [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Credit quality indicator by asset risk classification	3,228,538	3,161,593
Non-Covered Legacy Loans [Member] | Pass [Member] | Commercial Business [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Credit quality indicator by asset risk classification	3,222,102	3,245,912
Non-Covered Legacy Loans [Member] | Special Mention [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Credit quality indicator by asset risk classification	77,426	57,417
Non-Covered Legacy Loans [Member] | Special Mention [Member] | Commercial Real Estate Construction [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Credit quality indicator by asset risk classification	1,413	240
Non-Covered Legacy Loans [Member] | Special Mention [Member] | Commercial Real Estate - Other [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Credit quality indicator by asset risk classification	53,850	49,847
Non-Covered Legacy Loans [Member] | Special Mention [Member] | Commercial Business [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Credit quality indicator by asset risk classification	22,163	7,330
Non-Covered Legacy Loans [Member] | Sub-standard [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Credit quality indicator by asset risk classification	60,569	51,251
Non-Covered Legacy Loans [Member] | Sub-standard [Member] | Commercial Real Estate Construction [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Credit quality indicator by asset risk classification	440	69
Non-Covered Legacy Loans [Member] | Sub-standard [Member] | Commercial Real Estate - Other [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Credit quality indicator by asset risk classification	28,017	22,217
Non-Covered Legacy Loans [Member] | Sub-standard [Member] | Commercial Business [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Credit quality indicator by asset risk classification	32,112	28,965
Non-Covered Legacy Loans [Member] | Doubtful [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Credit quality indicator by asset risk classification	2,097	2,095
Non-Covered Legacy Loans [Member] | Doubtful [Member] | Commercial Real Estate - Other [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Credit quality indicator by asset risk classification	208	162
Non-Covered Legacy Loans [Member] | Doubtful [Member] | Commercial Business [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Credit quality indicator by asset risk classification	1,889	1,933
Non-Covered Acquired Loans [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Discount	(2,772,925)	(1,327,786)
Credit risk by payment status, Current	2,742,427	1,326,950
Total	2,772,925	1,327,786
Non-Covered Acquired Loans [Member] | Fair Value [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Credit risk by payment status, Current	1,008,715	729,570
30+ Days Past Due	33,966	25,225
Discount	(30,915)	(18,507)
Total	1,011,766	736,288
Non-Covered Acquired Loans [Member] | Fair Value [Member] | Residential Prime [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Credit risk by payment status, Current	477,973	418,552
30+ Days Past Due	16,779	11,858
Discount	(16,048)	(5,831)
Total	478,704	424,579
Non-Covered Acquired Loans [Member] | Fair Value [Member] | Home Equity [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Credit risk by payment status, Current	438,938	216,310
30+ Days Past Due	14,507	9,808
Discount	(11,805)	(8,419)
Total	441,640	217,699
Non-Covered Acquired Loans [Member] | Fair Value [Member] | Consumer - Other [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Credit risk by payment status, Current	91,527	94,315
30+ Days Past Due	2,655	3,520
Discount	(3,032)	(4,217)
Total	91,150	93,618
Non-Covered Acquired Loans [Member] | Fair Value [Member] | Indirect Automobile [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Credit risk by payment status, Current	277	393
30+ Days Past Due	25	39
Discount	(30)	(40)
Total	272	392
Non-Covered Acquired Loans [Member] | Internally Assigned Grade Loss [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Discount	(44,561)	(45,513)
Total	1,761,159
Non-Covered Acquired Loans [Member] | Internally Assigned Grade Loss [Member] | Commercial Real Estate Construction [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Discount	(987)	(1,170)
Total	61,676
Non-Covered Acquired Loans [Member] | Internally Assigned Grade Loss [Member] | Commercial Real Estate - Other [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Discount	(39,931)	(39,119)
Total	1,237,888
Non-Covered Acquired Loans [Member] | Internally Assigned Grade Loss [Member] | Commercial Business [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Discount	(3,643)	(5,224)
Total	461,595
Non-Covered Acquired Loans [Member] | Pass [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Credit quality indicator by asset risk classification	1,691,826	561,512
Non-Covered Acquired Loans [Member] | Pass [Member] | Commercial Real Estate Construction [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Credit quality indicator by asset risk classification	54,705	24,118
Non-Covered Acquired Loans [Member] | Pass [Member] | Commercial Real Estate - Other [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Credit quality indicator by asset risk classification	1,183,950	445,557
Non-Covered Acquired Loans [Member] | Pass [Member] | Commercial Business [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Credit quality indicator by asset risk classification	453,171	91,837
Non-Covered Acquired Loans [Member] | Special Mention [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Credit quality indicator by asset risk classification	22,792	16,661
Non-Covered Acquired Loans [Member] | Special Mention [Member] | Commercial Real Estate Construction [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Credit quality indicator by asset risk classification	602	2,006
Non-Covered Acquired Loans [Member] | Special Mention [Member] | Commercial Real Estate - Other [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Credit quality indicator by asset risk classification	18,391	12,794
Non-Covered Acquired Loans [Member] | Special Mention [Member] | Commercial Business [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Credit quality indicator by asset risk classification	3,799	1,861
Non-Covered Acquired Loans [Member] | Sub-standard [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Credit quality indicator by asset risk classification	88,226	58,581
Non-Covered Acquired Loans [Member] | Sub-standard [Member] | Commercial Real Estate Construction [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Credit quality indicator by asset risk classification	6,447	4,624
Non-Covered Acquired Loans [Member] | Sub-standard [Member] | Commercial Real Estate - Other [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Credit quality indicator by asset risk classification	74,368	49,139
Non-Covered Acquired Loans [Member] | Sub-standard [Member] | Commercial Business [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Credit quality indicator by asset risk classification	7,411	4,818
Non-Covered Acquired Loans [Member] | Doubtful [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Credit quality indicator by asset risk classification	2,842	257
Non-Covered Acquired Loans [Member] | Doubtful [Member] | Commercial Real Estate Construction [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Credit quality indicator by asset risk classification	909
Non-Covered Acquired Loans [Member] | Doubtful [Member] | Commercial Real Estate - Other [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Credit quality indicator by asset risk classification	1,110
Non-Covered Acquired Loans [Member] | Doubtful [Member] | Commercial Business [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Credit quality indicator by asset risk classification	823	257
Non-Covered Acquired Loans [Member] | Loss [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Credit quality indicator by asset risk classification	34
Non-Covered Acquired Loans [Member] | Loss [Member] | Commercial Business [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Credit quality indicator by asset risk classification	$ 34