Changes in Pension and Other Postretirement Benefit Plans (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Other Postretirement Benefit Plans, Defined Benefit
Change in Projected Benefit Obligation [Abstract]
Accumulated postretirement benefit obligation at beginning of year	$ 4,635	$ 4,339
Service cost	161	164	177
Interest cost	170	222	219
Benefit payments	(193)	(100)
Actuarial loss	(394)	10
Accumulated postretirement benefit obligation at end of year	4,379	4,635	4,339
Defined Benefit Plan, Amounts Recognized in Balance Sheet [Abstract]
Funded status – net prepaid	4,379	4,635
Pension and Other Postretirement Defined Benefit Plans, Current Liabilities	(296)	(218)
Pension and Other Postretirement Defined Benefit Plans, Liabilities, Noncurrent	(4,083)	(4,417)
Net amount recognized	(4,379)	(4,635)
Pension and Other Postretirement Benefit Plans, Accumulated Other Comprehensive Income (Loss), before Tax [Abstract]
Pension and Other Postretirement Benefit Plans, Accumulated Other Comprehensive Income (Loss), Net Gains (Losses), before Tax	661	266
Pension and Other Postretirement Benefit Plans, Accumulated Other Comprehensive Income (Loss), before Tax	661	266
Amortization of prior service cost	0	0	29
Pension Plans
Change in Projected Benefit Obligation [Abstract]
Projected benefit obligation at beginning of year	164,407	144,235
Accumulated postretirement benefit obligation at beginning of year	163,874
Service cost	608	539	623
Interest cost	6,832	7,393	7,456
Benefit payments	(6,558)	(6,151)
Actuarial loss	15,848	18,391
Projected benefit obligation at end of year	181,137	164,407	144,235
Accumulated postretirement benefit obligation at end of year	180,551	163,874
Change in Plan Assets [Abstract]
Fair value of plan assets at beginning of year	174,938	157,996
Defined Benefit Plan, Actual Return on Plan Assets	18,463	22,867
Employer contributions	307	226
Fair value of plan assets at end of year	187,150	174,938	157,996
Defined Benefit Plan, Amounts Recognized in Balance Sheet [Abstract]
Funded status – net prepaid	6,013	10,531
Defined Benefit Plan, Assets for Plan Benefits	12,891	15,956
Accrued liabilities	(309)	(219)
Pension benefit obligations	(6,569)	(5,206)
Net amount recognized	6,013	10,531
Pension and Other Postretirement Benefit Plans, Accumulated Other Comprehensive Income (Loss), before Tax [Abstract]
Pension and Other Postretirement Benefit Plans, Accumulated Other Comprehensive Income (Loss), Net Gains (Losses), before Tax	(29,778)	(23,131)
Pension and Other Postretirement Benefit Plans, Accumulated Other Comprehensive Income (Loss), Net Prior Service Cost (Credit), before Tax	(1,617)	(1,942)
Pension and Other Postretirement Benefit Plans, Accumulated Other Comprehensive Income (Loss), before Tax	(31,395)	(25,073)
Amortization of prior service cost	$ 324	$ 324	$ 231