Consolidated Statements of Financial Position (Parenthetical) (USD $) In Thousands, except Share data, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012	Mar. 31, 2012
Held-to-maturity, fair value	$ 261,405	$ 229,922	$ 192,372
Accumulated depreciation of premises and equipment	$ 94,068	$ 93,207	$ 88,603
Preferred stock, no par value
Preferred stock, shares authorized	200,000	200,000	200,000
Preferred stock, shares issued	0	0	0
Common stock, par value	$ 1	$ 1	$ 1
Common stock, shares authorized	45,000,000	45,000,000	45,000,000
Common stock, shares issued	27,532,377	27,498,868	27,491,026
Common stock, shares outstanding	27,532,377	27,498,868	27,491,026