Pension Plan and Other Postretirement Benefits (Details) (USD $)			3 Months Ended		6 Months Ended		3 Months Ended		6 Months Ended
	Jun. 30, 2011	Dec. 31, 2010	Jun. 30, 2011 Pension Benefits [Member]	Jun. 30, 2010 Pension Benefits [Member]	Jun. 30, 2011 Pension Benefits [Member]	Jun. 30, 2010 Pension Benefits [Member]	Jun. 30, 2011 Other Postretirement Benefits [Member]	Jun. 30, 2010 Other Postretirement Benefits [Member]	Jun. 30, 2011 Other Postretirement Benefits [Member]	Jun. 30, 2010 Other Postretirement Benefits [Member]
Service cost			$ 2,331,000	$ 1,774,000	$ 4,662,000	$ 3,548,000	$ 339,000	$ 272,000	$ 677,000	$ 544,000
Interest cost			918,000	812,000	1,835,000	1,624,000	63,000	49,000	126,000	98,000
Expected return on plan assets			(1,100,000)	(876,000)	(2,199,000)	(1,752,000)
Amortization of transition obligation							16,000	16,000	32,000	32,000
Amortization of prior service cost			86,000	87,000	172,000	173,000	3,000	4,000	7,000	8,000
Amortization of net loss			214,000	202,000	428,000	404,000	3,000	5,000	6,000	10,000
Net periodic benefit cost			2,449,000	1,999,000	4,898,000	3,997,000	424,000	346,000	848,000	692,000
Defined benefit plan, Expected employer contributions			12,500,000		12,500,000		100,000		100,000
Employer contributions					$ 5,900,000				$ 100,000
Common stock purchased under non-qualified plan, Treasury stock, Shares	125,550	156,636