Premises and Equipment (Details) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Premises and Equipment
Cost	$ 560,022,000	$ 522,054,000
Accumulated Depreciation And Amortization	352,322,000	323,656,000
Carrying Value	207,700,000	198,398,000
Depreciation and Amortization Excluding Amortization of Intangible Assets	32,421,000	32,267,000	32,485,000
Net rental payments on operating leases	54,200,000	56,400,000	52,500,000
Premises, including land
Premises and Equipment
Cost	215,123,000	206,313,000
Accumulated Depreciation And Amortization	124,805,000	114,330,000
Carrying Value	90,318,000	91,983,000
Land	15,951,000	15,011,000
Premises, including land | Maximum
Premises and Equipment
Range of Lives	39 years	39 years
Furniture, fixtures and equipment
Premises and Equipment
Cost	173,257,000	163,969,000
Accumulated Depreciation And Amortization	136,663,000	129,142,000
Carrying Value	36,594,000	34,827,000
Furniture, fixtures and equipment | Minimum
Premises and Equipment
Range of Lives	3 years	3 years
Furniture, fixtures and equipment | Maximum
Premises and Equipment
Range of Lives	10 years	10 years
Software
Premises and Equipment
Cost	171,642,000	151,772,000
Accumulated Depreciation And Amortization	90,854,000	80,184,000
Carrying Value	$ 80,788,000	$ 71,588,000
Software | Minimum
Premises and Equipment
Range of Lives	3 years	5 years
Software | Maximum
Premises and Equipment
Range of Lives	10 years	10 years