Fair Value Measurements (Narrative) (Detail) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total credit-related financial instruments with off-balance sheet risk	$ 6,110,820	$ 6,050,806
Fair Value, Nonrecurring [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Book value of other real estate owned (OREO) and repossessed assets	4,000
Rabbi Trust [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Other assets	1,700
Alternative investments [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total credit-related financial instruments with off-balance sheet risk	25,000
Alternative investments [Member] | Fair Value, Nonrecurring [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Other assets	$ 6,900