Commitments and Contingent Liabilities - Commitments to Extend Credit, Guarantees and Various Letters of Credit Outstanding (Detail) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Commitments and Contingencies Disclosure [Abstract]
Lines of credit	$ 1,571,547	$ 1,452,697
Loans approved but not closed	354,696	245,644
Overdraft limits	126,101	126,671
Letters of credit	38,716	31,951
Contingent obligations and other guarantees	$ 5,818	$ 6,700