Postretirement Benefits (Details) (Supplemental nonqualified retirement plan, USD $)	3 Months Ended
	Mar. 31, 2013	Mar. 31, 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		$ 13,100,000
Net periodic benefit (income) cost of plans	(481,000)	(706,000)
Interest costs	214,000	200,000
Service costs	10,000	30,000
Investment gain (loss)	705,000	936,000
Projected benefit obligations	$ 12,900,000		$ 12,700,000
Weighted-average discount rate (as a percent)	6.80%	7.00%