Pension Plan and Postretirement Benefits Other Than Pension (Details) (USD $)	3 Months Ended			9 Months Ended
	Sep. 30, 2011	Mar. 31, 2011	Sep. 30, 2010	Sep. 30, 2011 Y	Sep. 30, 2010
Pension Benefits
Pension Plan and Postretirement Benefits Other than Pension
Final number of years of employees compensation to determine the benefits payable (in years)				10
Components of net periodic benefit cost
Service cost	$ 1,775,000		$ 1,535,000	$ 5,326,000	$ 4,605,000
Interest cost	1,799,000		1,649,000	5,397,000	4,947,000
Expected return on plan assets	(2,191,000)		(1,875,000)	(6,573,000)	(5,624,000)
Actuarial loss amortization	451,000		404,000	1,353,000	1,212,000
Prior service cost amortization	139,000		139,000	417,000	417,000
Transition obligation amortization	1,000		1,000	3,000	3,000
Total	1,974,000		1,853,000	5,923,000	5,560,000
Employer contribution to the pension plan		10,000,000
Other Postretirement Benefits
Pension Plan and Postretirement Benefits Other than Pension
Age of employees after which the plan does not provide benefits (in years)				65
Components of net periodic benefit cost
Service cost	139,000		111,000	418,000	332,000
Interest cost	100,000		91,000	301,000	273,000
Prior service cost amortization	14,000		11,000	42,000	34,000
Total	$ 253,000		$ 213,000	$ 761,000	$ 639,000