Premises and Equipment (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Property, Plant and Equipment
Premises and equipment, gross	$ 404,200	$ 346,700
Less accumulated depreciation	(162,300)	(152,200)
Premises and equipment, net	241,862	194,457
Land
Property, Plant and Equipment
Premises and equipment, gross	49,400	39,800
Buildings and improvements
Property, Plant and Equipment
Premises and equipment, gross	257,100	218,300
Furniture and equipment
Property, Plant and Equipment
Premises and equipment, gross	$ 97,700	$ 88,600