Pension and Other Postretirement Benefit Plans (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011
Defined Benefit Pension Plans [Member]
Components of net periodic benefit cost (income) for the Corporation's qualified and nonqualified pension plans and nonqualified postretirement benefits plan
Service cost	$ 304	$ 290	$ 910	$ 872
Interest cost	1,214	1,207	3,644	3,622
Expected return on plan assets	(1,731)	(1,582)	(5,194)	(4,746)
Amortization of prior service credit	0	0	(1)	(1)
Amortization of unrecognized net loss (gain)	614	270	1,843	809
Net periodic benefit cost (income)	401	185	1,202	556
Postretirement Benefit Plan [Member]
Components of net periodic benefit cost (income) for the Corporation's qualified and nonqualified pension plans and nonqualified postretirement benefits plan
Interest cost	37	42	110	125
Amortization of prior service credit	(75)	(81)	(225)	(243)
Amortization of unrecognized net loss (gain)	(7)	(8)	(21)	(24)
Net periodic benefit cost (income)	$ (45)	$ (47)	$ (136)	$ (142)