Additional Information Regarding Methods used to Evaluate Loan Portfolio for Impairment (Detail) - Additional Information Loan Portfolio - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans Receivable, Individually evaluated for impairment	$ 80,557	$ 48,252
Loans Receivable, Collectively evaluated for impairment	38,278,465	37,302,187
Total loans, net	38,359,022	39,054,334
Acquired loans with deteriorated credit quality
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans Receivable, other		1,703,895
Mortgage Receivable
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans Receivable, Individually evaluated for impairment	31,747	29,660
Loans Receivable, Collectively evaluated for impairment	36,278,241	35,402,029
Total loans, net	36,309,988	37,046,444
Mortgage Receivable | Acquired loans with deteriorated credit quality
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans Receivable, other		1,614,755
Other loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans Receivable, Individually evaluated for impairment	48,810	18,592
Loans Receivable, Collectively evaluated for impairment	2,000,224	1,900,158
Total loans, net	$ 2,049,034	2,007,890
Other loans | Acquired loans with deteriorated credit quality
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans Receivable, other		$ 89,140