LOAN PORTFOLIO- Corporation's Aging of Purchased Credit Impaired Loans (Detail) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018	Mar. 31, 2018	Dec. 31, 2017
Accounts Notes And Loans Receivable [Line Items]
Total Past Due	$ 529,279	$ 574,654
Financing Receivable, Current	8,323,094	8,136,829
Loans held for investment	8,996,816	8,858,123	$ 8,695,890
Residential Portfolio Segment [Member]
Accounts Notes And Loans Receivable [Line Items]
Loans held for investment	3,126,562	3,163,208	3,267,868
Commercial Portfolio Segment [Member]
Accounts Notes And Loans Receivable [Line Items]
Loans held for investment	3,854,962	3,750,202
Commercial Portfolio Segment [Member] | Mortgage Receivable [Member]
Accounts Notes And Loans Receivable [Line Items]
Total Past Due	95,783	111,520
Financing Receivable, Current	1,459,477	1,407,678
Loans held for investment	1,558,724	1,522,662	1,552,503
PCI loans
Accounts Notes And Loans Receivable [Line Items]
Total Past Due	35,650	36,423
Financing Receivable, Current	108,793	110,217
Loans held for investment	144,443	146,640	155,281	$ 158,174
PCI loans | Residential Portfolio Segment [Member]
Accounts Notes And Loans Receivable [Line Items]
Total Past Due	33,133	33,911
Financing Receivable, Current	107,846	109,265
Loans held for investment	140,979	143,176	151,067
PCI loans | Commercial Portfolio Segment [Member] | Mortgage Receivable [Member]
Accounts Notes And Loans Receivable [Line Items]
Total Past Due	2,517	2,512
Financing Receivable, Current	947	952
Loans held for investment	3,464	3,464	$ 4,214
30-59 Days Past Due [Member]
Accounts Notes And Loans Receivable [Line Items]
Total Past Due	48,902	49,020
30-59 Days Past Due [Member] | Commercial Portfolio Segment [Member] | Mortgage Receivable [Member]
Accounts Notes And Loans Receivable [Line Items]
Total Past Due	0	0
Loans held for investment	6,000	5,100
30-59 Days Past Due [Member] | PCI loans
Accounts Notes And Loans Receivable [Line Items]
Total Past Due	0	0
30-59 Days Past Due [Member] | PCI loans | Residential Portfolio Segment [Member]
Accounts Notes And Loans Receivable [Line Items]
Total Past Due	0	0
Loans held for investment	12,800	11,600
30-59 Days Past Due [Member] | PCI loans | Commercial Portfolio Segment [Member] | Mortgage Receivable [Member]
Accounts Notes And Loans Receivable [Line Items]
Total Past Due	0	0
Loans held for investment	0	0
60-89 Days Past Due [Member]
Accounts Notes And Loans Receivable [Line Items]
Total Past Due	87,395	80,578
60-89 Days Past Due [Member] | Commercial Portfolio Segment [Member] | Mortgage Receivable [Member]
Accounts Notes And Loans Receivable [Line Items]
Total Past Due	1,371	1,038
60-89 Days Past Due [Member] | PCI loans
Accounts Notes And Loans Receivable [Line Items]
Total Past Due	7,490	6,979
60-89 Days Past Due [Member] | PCI loans | Residential Portfolio Segment [Member]
Accounts Notes And Loans Receivable [Line Items]
Total Past Due	7,490	6,979
60-89 Days Past Due [Member] | PCI loans | Commercial Portfolio Segment [Member] | Mortgage Receivable [Member]
Accounts Notes And Loans Receivable [Line Items]
Total Past Due	0	0
90 days or more Past Due [Member]
Accounts Notes And Loans Receivable [Line Items]
Total Past Due	392,982	445,056
90 days or more Past Due [Member] | Commercial Portfolio Segment [Member] | Mortgage Receivable [Member]
Accounts Notes And Loans Receivable [Line Items]
Total Past Due	94,412	110,482
90 days or more Past Due [Member] | PCI loans
Accounts Notes And Loans Receivable [Line Items]
Total Past Due	28,160	29,444
90 days or more Past Due [Member] | PCI loans | Residential Portfolio Segment [Member]
Accounts Notes And Loans Receivable [Line Items]
Total Past Due	25,643	26,932
90 days or more Past Due [Member] | PCI loans | Commercial Portfolio Segment [Member] | Mortgage Receivable [Member]
Accounts Notes And Loans Receivable [Line Items]
Total Past Due	$ 2,517	$ 2,512