Premises and Equipment - Components of Premises and Equipment (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Property Plant And Equipment [Line Items]
Cost	$ 520,825	$ 500,488
Accumulated Depreciation and Amortization	(321,130)	(312,100)
Net Book Value	199,695	188,388
Premises
Property Plant And Equipment [Line Items]
Cost	370,268	360,170
Accumulated Depreciation and Amortization	(237,024)	(235,031)
Net Book Value	133,244	125,139
Equipment
Property Plant And Equipment [Line Items]
Cost	143,964	133,725
Accumulated Depreciation and Amortization	(79,817)	(72,852)
Net Book Value	64,147	60,873
Finance leases
Property Plant And Equipment [Line Items]
Cost	6,593	6,593
Accumulated Depreciation and Amortization	(4,289)	(4,217)
Net Book Value	$ 2,304	$ 2,376