Premises and Equipment, Net of Accumulated Depreciation (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Premises and Equipment, Net of Accumulated Depreciation
Accumulated depreciation	$ (99,409)	$ (92,944)
Premises and equipment, net	179,175	167,671
Land
Premises and Equipment, Net of Accumulated Depreciation
Premises and equipment, gross	27,605	27,260
Office buildings and construction in progress
Premises and Equipment, Net of Accumulated Depreciation
Premises and equipment, gross	172,544	159,391
Furniture, fixtures and equipment
Premises and Equipment, Net of Accumulated Depreciation
Premises and equipment, gross	70,622	66,375
Leasehold improvements
Premises and Equipment, Net of Accumulated Depreciation
Premises and equipment, gross	$ 7,813	$ 7,589