Supplemental Executive Retirement Plan (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011	Dec. 31, 2011
Supplemental Executive Retirement Plan Disclosure
Marketable investments and cash-restricted	$ 5,348		$ 5,348		$ 5,123
Supplemental Executive Retirement Plan (SERP)
Supplemental Executive Retirement Plan Disclosure
Marketable investments and cash-restricted	5,100		5,100		5,100
Components of net periodic pension cost
Service cost	1,078	1,079	3,236	3,176
Interest cost	369	352	1,106	1,004
Amoritization of prior service costs	207	207	620	566
Amortization of net actuarial loss		20		71
Net pension expense	$ 1,654	$ 1,658	$ 4,962	$ 4,817