Pension Plan and Other Postretirement Benefits (Details) (USD $)			3 Months Ended			3 Months Ended
	Mar. 31, 2012	Dec. 31, 2011	Mar. 31, 2012 Pension Benefits [Member]	Mar. 31, 2011 Pension Benefits [Member]	Dec. 31, 2012 Pension Benefits [Member]	Mar. 31, 2012 Other Postretirement Benefits [Member]	Mar. 31, 2011 Other Postretirement Benefits [Member]	Dec. 31, 2012 Other Postretirement Benefits [Member]
Service cost			$ 2,736,000	$ 2,331,000		$ 458,000	$ 338,000
Interest cost			1,013,000	917,000		100,000	63,000
Expected return on plan assets			(1,357,000)	(1,099,000)
Amortization of transition obligation						16,000	16,000
Amortization of prior service cost			71,000	86,000		3,000	4,000
Amortization of net loss			305,000	214,000		23,000	3,000
Net periodic benefit cost			2,768,000	2,449,000		600,000	424,000
Defined benefit plan, Expected employer contributions					16,200,000			100,000
Employer contributions			$ 3,000,000			$ 100,000
Common stock purchased under non-qualified plan, Treasury stock, Shares	100,970	98,889