Employee Benefit Plans (Details) (USD $)	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012	Dec. 31, 2012
Components of net periodic benefit cost [Abstract]
Cost recognized	$ 39,000	$ 33,000	$ 77,000	$ 66,000
Employer contribution to 401 (k) defined contribution plan	92,000	80,000	147,000	132,000
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares authorized under restricted stock plan (in shares)	100,000		100,000
Number of restricted stock available for grant (in shares)	67,756		67,756
Restricted Stock [Member]
Restricted stock activity [Roll Forward]
Outstanding, beginning of period	7,269	6,280	8,646	9,921
Granted	3,027	3,808	3,027	3,808
Forfeited	0	0	(55)	0
Vested	(2,830)	(1,581)	(4,152)	(5,222)
Outstanding, end of period	7,466	8,507	7,466	8,507
Weighted average market price [Roll Forward]
Outstanding, beginning of period	$ 35.16	$ 28.15	$ 35.51	$ 29.37
Granted	$ 48.21	$ 37.10	$ 48.21	$ 37.10
Forfeited	$ 0	$ 0	$ 37.10	$ 0
Vested	$ 31.35	$ 26.80	$ 33.26	$ 25.59
Outstanding, end of period	$ 41.89	$ 35.16	$ 41.89	$ 35.16
Directors [Member]
Deferred Compensation Arrangement with Individual, Postretirement Benefits [Line Items]
Deferred interest expense	3,000	4,000	7,000	8,000
Pension Plan [Member]
Components of net periodic benefit cost [Abstract]
Service cost	76,000	54,000	177,000	167,000
Interest cost	79,000	35,000	185,000	174,000
Expected return on plan assets	(169,000)	(80,000)	(343,000)	(286,000)
Net amortization and deferral	86,000	52,000	128,000	68,000
Net periodic benefit cost	72,000	61,000	147,000	123,000
Expected employer contribution to pension plan	1,000,000		1,000,000
Supplemental Executive Retirement Plans [Member]
Components of net periodic benefit cost [Abstract]
Obligation included in other liabilities	973,000		973,000		901,000
Cost recognized	36,000	24,000	72,000	47,000
Deferred Compensation Plan [Member]
Components of net periodic benefit cost [Abstract]
Obligation included in other liabilities	$ 971,000		$ 971,000		$ 1,001,000