Bank Premises and Equipment - Summary of Bank Premises and Equipment (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Property, Plant and Equipment [Line Items]
Cost	$ 112,787	$ 101,843
Accumulated Depreciation & Amortization	(45,904)	(43,159)
Net Carrying Value	66,883	58,684
Premises [Member]
Property, Plant and Equipment [Line Items]
Cost	78,255	71,562
Accumulated Depreciation & Amortization	(24,045)	(22,969)
Net Carrying Value	54,210	48,593
Furniture and equipment [Member]
Property, Plant and Equipment [Line Items]
Cost	34,532	30,281
Accumulated Depreciation & Amortization	(21,859)	(20,190)
Net Carrying Value	$ 12,673	$ 10,091