Premises and Equipment - Components and Depreciation and Amortization Expenses (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Components of premises and equipment
Premises and equipment	$ 495,181	$ 498,216
Less: Accumulated depreciation and amortization	205,966	197,428
Net book value	289,215	300,788
Depreciation and amortization expenses	15,278	14,799	$ 14,546
Occupancy
Components of premises and equipment
Depreciation and amortization expenses	8,852	8,901	9,039
Equipment
Components of premises and equipment
Depreciation and amortization expenses	6,426	5,898	$ 5,507
Buildings
Components of premises and equipment
Premises and equipment	279,635	279,563
Furniture and equipment
Components of premises and equipment
Premises and equipment	80,519	76,832
Land
Components of premises and equipment
Premises and equipment	88,031	89,164
Leasehold improvements
Components of premises and equipment
Premises and equipment	$ 46,996	$ 52,657