Unaudited Interim Financial Data (Details) (USD $)
In Thousands, except Per Share data, unless otherwise specified
 3 Months Ended 12 Months Ended
Dec. 31, 2014
Sep. 30, 2014
Jun. 30, 2014
Mar. 31, 2014
Dec. 31, 2013
Sep. 30, 2013
Jun. 30, 2013
Mar. 31, 2013
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Unaudited Interim Financial Data                      
Interest income: $ 27,657 $ 27,511 $ 26,656 $ 26,251 $ 26,885 $ 27,236 $ 27,460 $ 27,115 $ 108,075 $ 108,696 $ 116,916
Interest expense: 1,570 1,600 1,635 1,694 1,881 2,003 2,215 2,532 6,499 8,631 14,770
Net interest income 26,087 25,911 25,021 24,557 25,004 25,233 25,245 24,583 101,576 100,065 102,146
Provision for loan losses     1,000 1,000 1,500 2,000 2,000 2,000 2,000 7,500 16,500
Other income: 14,739 14,204 15,012 14,986 14,529 15,630 15,783 16,641 58,941 62,583 66,852
Non-interest expense 28,470 26,298 26,823 26,618 27,585 27,358 27,801 29,567 108,209 112,311 119,644
Income before income taxes 12,356 13,817 12,210 11,925 10,448 11,505 11,227 9,657 50,308 42,837 32,854
Income taxes 4,763 4,708 4,025 4,038 3,528 3,572 3,787 3,224 17,534 14,111 10,497
Net income 7,593 9,109 8,185 7,887 6,920 7,933 7,440 6,433 32,774 28,726 22,357
Preferred stock dividends 182 182 181 182 908 909 908 908 727 3,633 3,633
Net income available for common stockholders $ 7,411 $ 8,927 $ 8,004 $ 7,705 $ 6,012 $ 7,024 $ 6,532 $ 5,525 $ 32,047 $ 25,093 $ 18,724
Basic earnings per share (in dollars per share) $ 0.08 $ 0.10 $ 0.09 $ 0.09 $ 0.07 $ 0.08 $ 0.08 $ 0.06 $ 0.37 $ 0.29 $ 0.22
Diluted earnings per share (in dollars per share) $ 0.08 $ 0.10 $ 0.09 $ 0.09 $ 0.07 $ 0.08 $ 0.08 $ 0.06 $ 0.37 $ 0.29 $ 0.22