Condensed Consolidated Balance Sheets (unaudited) (Parenthetical) (USD $) In Thousands, except Share data, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Assets:
Securities available-for-sale at fair value (amortized cost)	$ 660,611	$ 583,858
Securities held-to-maturity at amortized cost (fair value)	1,656	1,659
Core deposit intangible (accumulated amortization)	$ 7,765	$ 7,712
Shareholders' equity:
Preferred stock, shares authorized (in shares)	300,000	300,000
Preferred stock, shares unissued (in shares)	300,000	300,000
Common stock, par value (in dollars per share)	$ 5	$ 5
Common stock, shares authorized (in shares)	25,000,000	25,000,000
Common stock, shares outstanding (in shares)	15,653,090	15,612,935