Loans and Allowance for Credit Losses - Schedule of Past Due Financing Receivables (Detail) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Financing Receivable, Recorded Investment, Past Due [Line Items]
30-59 Days Past Due	$ 20,971	$ 22,461
60-89 Days Past Due	6,094	8,024
Recorded Investment > 90 Days and Accruing	142	458
Nonaccrual	155,892	140,857
Total Past Due	183,099	171,800
Current	6,469,440	6,146,401
Commercial Loan [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
30-59 Days Past Due	2,486	649
60-89 Days Past Due	1,774	813
Recorded Investment > 90 Days and Accruing		33
Nonaccrual	51,683	38,460
Total Past Due	55,943	39,955
Current	1,612,332	1,589,645
Commercial Real Estate - Construction [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
30-59 Days Past Due	927
Nonaccrual	6,195	6,499
Total Past Due	7,122	6,499
Current	143,589	128,053
Commercial Real Estate - Other [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
30-59 Days Past Due	2,271	3,834
60-89 Days Past Due	2,096	1,468
Recorded Investment > 90 Days and Accruing		138
Nonaccrual	65,203	60,903
Total Past Due	69,570	66,343
Current	1,593,298	1,510,215
Consumer Heloc [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
30-59 Days Past Due	839	577
60-89 Days Past Due	140	376
Nonaccrual	3,073	3,115
Total Past Due	4,052	4,068
Current	370,027	356,252
Consumer Auto [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
30-59 Days Past Due	2,441	3,349
60-89 Days Past Due	490	695
Recorded Investment > 90 Days and Accruing	83	203
Nonaccrual	1,263	1,261
Total Past Due	4,277	5,508
Current	892,913	841,461
Consumer - Other [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
30-59 Days Past Due	727	969
60-89 Days Past Due	223	129
Recorded Investment > 90 Days and Accruing	44	83
Nonaccrual	1,510	1,527
Total Past Due	2,504	2,708
Current	134,718	100,630
Residential [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
30-59 Days Past Due	10,191	11,606
60-89 Days Past Due	847	3,959
Recorded Investment > 90 Days and Accruing		1
Nonaccrual	14,422	13,968
Total Past Due	25,460	29,534
Current	1,599,894	1,489,622
Covered Loans [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
30-59 Days Past Due	1,089	1,477
60-89 Days Past Due	524	584
Recorded Investment > 90 Days and Accruing	15
Nonaccrual	12,543	15,124
Total Past Due	14,171	17,185
Current	$ 122,669	$ 130,523