Postretirement Benefits (Details) (Supplemental nonqualified retirement plan, USD $)	3 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012	Dec. 31, 2012
Supplemental nonqualified retirement plan
Employee benefit plan
Retirement plan vesting period			5 years
Value of assets held in trust	$ 14,500,000		$ 14,500,000		$ 13,100,000
Net periodic benefit income of plans	427,000	969,000	723,000	815,000
Death benefit		863,000		863,000
Interest costs	215,000	301,000	644,000	903,000
Service costs	11,000	30,000	31,000	90,000
Investment gain (loss)	653,000	437,000	1,400,000	945,000
Projected benefit obligations	$ 13,300,000		$ 13,300,000		$ 12,700,000
Weighted-average discount rate (as a percent)	6.80%	6.60%	6.80%	6.60%