Parent Company Only Financial Information (Details 3) (USD $)
In Thousands, 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
Cash Flows from Operating Activities                      
Net income $ 7,593 $ 9,109 $ 8,185 $ 7,887 $ 6,920 $ 7,933 $ 7,440 $ 6,433 $ 32,774 $ 28,726 $ 22,357
Adjustments to reconcile net income to net cash provided by (used in) operating activities:                      
Depreciation and amortization                 8,456 8,598 8,759
Stock-based compensation                 1,187 1,079 991
Fair value adjustment on employee stock ownership plan shares allocated                     (322)
Changes in assets and liabilities:                      
Decrease (increase) in other assets                 2,274 9,880 8,368
(Decrease) increase in other liabilities                 (650) (63) 992
Net cash provided by operating activities                 68,111 98,153 49,069
Cash Flows from Investing Activities                      
Decrease in loans                 (126,604) (258,366) (33,680)
Proceeds from sales of premises and equipment                 78 2,856 77
Purchases of premises and equipment                 (3,778) (2,549) (8,611)
Net cash used in investing activities                 (51,262) (118,698) (204,435)
Cash Flows from Financing Activities                      
Value of shares surrendered upon vesting of restricted stock units to cover tax obligations                 (45) (237)  
Cash dividends paid                 (17,224) (14,040) (24,427)
Net cash provided by (used in) financing activities                 90,986 (99,107) 191,568
Cash and due from banks, beginning       231,603       351,255 231,603 351,255 315,053
Cash and due from banks, ending 339,438       231,603       339,438 231,603 351,255
Parent                      
Cash Flows from Operating Activities                      
Net income                 32,774 28,726 22,357
Adjustments to reconcile net income to net cash provided by (used in) operating activities:                      
Depreciation and amortization                 112 100 98
Distributions less than net income of subsidiaries                 (34,672) (28,658) (20,960)
Stock-based compensation                 1,187 1,079 991
Fair value adjustment on employee stock ownership plan shares allocated                     (322)
Changes in assets and liabilities:                      
Decrease (increase) in other assets                 951 (62) 125
(Decrease) increase in other liabilities                 (1,077) 381 1,435
Net cash provided by operating activities                 (725) 1,566 3,724
Cash Flows from Investing Activities                      
Decrease in loans                   68 95
Proceeds from sales of premises and equipment                   226  
Purchases of premises and equipment                 (114) (90) (11)
Net cash used in investing activities                 (114) 204 84
Cash Flows from Financing Activities                      
Proceeds from charter amendment with subsidiary bank                 60,000 50,000  
Value of shares surrendered upon vesting of restricted stock units to cover tax obligations                 (45) (237)  
Cash dividends paid                 (17,224) (14,040) (24,427)
Net cash provided by (used in) financing activities                 42,731 35,723 (24,427)
Net increase (decrease) in cash and due from subsidiary banks                 41,892 37,493 (20,619)
Cash and due from banks, beginning       42,645       5,152 42,645 5,152 25,771
Cash and due from banks, ending $ 84,537       $ 42,645       $ 84,537 $ 42,645 $ 5,152