Pension Plans and Other Postretirement Benefits (Components of Net Periodic Benefit Costs) (Details) (USD $) In Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2011	Jun. 30, 2010	Jun. 30, 2011	Jun. 30, 2010
Pension Benefits [Member]
Service cost	$ 1,044	$ 1,172	$ 2,087	$ 2,344
Interest cost	2,964	3,220	5,928	6,440
Expected return on plan assets	(2,596)	(2,796)	(5,191)	(5,592)
Amortization of prior service cost	43	35	87	70
Amortization of actuarial loss	1,016	1,030	2,033	2,060
Capitalized costs	(951)	(888)	(1,808)	(1,685)
Settlement charge		884		884
Net periodic benefit cost	1,520	2,657	3,136	4,521
Other Postretirement Benefits [Member]
Service cost	376	305	751	610
Interest cost	812	620	1,623	1,240
Expected return on plan assets	(621)	(462)	(1,243)	(924)
Amortization of transition obligation	34	26	69	52
Amortization of prior service cost	(89)	(67)	(178)	(134)
Amortization of actuarial loss	206	171	411	342
Amortization of regulatory asset	34	34	68	68
Capitalized costs	(188)	(129)	(343)	(250)
Net periodic benefit cost	$ 564	$ 498	$ 1,158	$ 1,004