Postretirement Benefits (Details) (Supplemental nonqualified retirement plan, USD $)	3 Months Ended		6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011	Dec. 31, 2011
Supplemental nonqualified retirement plan
Employee benefit plan
Retirement plan vesting period			5 years
Cash surrender value of insurance contracts	$ 11,700,000		$ 11,700,000		$ 11,100,000
Net periodic benefit cost of plans	860,000	223,000	154,000	367,000
Service costs	30,000	28,000	60,000	290,000
Interest costs	402,000	183,000	602,000	366,000
Investment gain (loss)	(428,000)	(12,000)	508,000	289,000
Projected benefit obligations	$ 12,000,000		$ 12,000,000		$ 11,300,000
Weighted-average discount rate (as a percent)	6.60%	7.00%	6.60%	7.00%