Fair Value Measurements (Narrative) (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jan. 01, 2019
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Unused Commitments to Extend Credit	$ 6,755,563	$ 6,379,941
ROU lease assets			$ 127,481	$ 157,234
Mortgage Loans in Process of Foreclosure, Amount	14,900
Valuation Allowance for Impairment of Recognized Servicing Assets, Sales and Disposals	1,900
Rabbi Trust [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Other Assets, Fair Value Disclosure	1,600
Alternative investments [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Other Assets, Fair Value Disclosure			$ 22,000
Fair Value, Nonrecurring [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
ROU lease assets	127,500
ROU assets impaired	1,500
Other Assets, Fair Value Disclosure	2,300
Fair Value, Nonrecurring [Member] | Alternative investments [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Unused Commitments to Extend Credit	17,400
Alternative investments, written up	$ 1,600