Postretirement Benefits (Details) (Supplemental nonqualified retirement plan, USD $)	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012	Dec. 31, 2012
Supplemental nonqualified retirement plan
Employee benefit plan
Retirement plan vesting period			5 years
Value of assets held in trust	$ 13,800,000		$ 13,800,000		$ 13,100,000
Net periodic benefit (income) cost of plans	185,000	860,000	(296,000)	154,000
Interest costs	215,000	402,000	429,000	602,000
Service costs	10,000	30,000	20,000	60,000
Investment gain (loss)	40,000	(428,000)	745,000	508,000
Projected benefit obligations	$ 13,100,000		$ 13,100,000		$ 12,700,000
Weighted-average discount rate (as a percent)	6.80%	6.60%	6.80%	6.60%