Retirement Plans (Details) (USD $)	3 Months Ended		3 Months Ended		6 Months Ended		12 Months Ended	3 Months Ended		6 Months Ended		12 Months Ended
	Mar. 31, 2011	Jun. 30, 2011	Jun. 30, 2011 Pension Plans, Defined Benefit [Member]	Jun. 30, 2010 Pension Plans, Defined Benefit [Member]	Jun. 30, 2011 Pension Plans, Defined Benefit [Member]	Jun. 30, 2010 Pension Plans, Defined Benefit [Member]	Dec. 31, 2010 Pension Plans, Defined Benefit [Member]	Jun. 30, 2011 Other Postretirement Benefit Plans, Defined Benefit [Member]	Jun. 30, 2010 Other Postretirement Benefit Plans, Defined Benefit [Member]	Jun. 30, 2011 Other Postretirement Benefit Plans, Defined Benefit [Member]	Jun. 30, 2010 Other Postretirement Benefit Plans, Defined Benefit [Member]	Dec. 31, 2010 Other Postretirement Benefit Plans, Defined Benefit [Member]
Net period benefit cost for the pension and postretirement plans
Service cost			$ 2,613,000	$ 2,475,000	$ 5,225,000	$ 4,950,000	$ 9,622,000
Interest cost			1,589,000	1,590,000	3,180,000	3,180,000	6,377,000	50,000	58,000	100,000	115,000	227,000
Expected return on plan assets			(3,220,000)	(3,019,000)	(6,440,000)	(6,038,000)	(12,152,000)
Amortization of prior service cost			18,000	18,000	35,000	35,000	72,000	99,000	99,000	198,000	198,000	395,000
Amortization of actuarial loss			451,000	405,000	903,000	810,000	1,601,000	0		0		(3,000)
Total net periodic benefit cost			1,451,000	1,469,000	2,903,000	2,937,000	5,520,000	149,000	157,000	298,000	313,000	619,000
Retirement Plans (Textuals) [Abstract]
Retirement eligible age for participation		At Least 55 years Of Age With 5 years Of Service
Contribution made by the Corporation to pension plan	$ 6,000,000