Changes in Pension and Other Postretirement Benefit Plans (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Other Postretirement Benefit Plans, Defined Benefit
Change in Projected Benefit Obligation [Abstract]
Accumulated postretirement benefit obligation at beginning of year	$ 1,977	$ 4,379
Service cost	56	153	161
Interest cost	76	148	170
Benefit payments	(224)	(201)
Actuarial loss (gain)	667	(2,502)
Accumulated postretirement benefit obligation at end of year	2,552	1,977	4,379
Defined Benefit Plan, Amounts Recognized in Balance Sheet [Abstract]
Funded status – net prepaid (liability)	(2,552)	(1,977)
Pension and Other Postretirement Defined Benefit Plans, Current Liabilities	(226)	(139)
Pension and Other Postretirement Defined Benefit Plans, Liabilities, Noncurrent	(2,326)	(1,838)
Net amount recognized	(2,552)	(1,977)
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	2,137	3,139
Pension and Other Postretirement Benefit Plans, Accumulated Other Comprehensive Income (Loss), before Tax	2,137	3,139
Pension Plans
Change in Projected Benefit Obligation [Abstract]
Accumulated postretirement benefit obligation at beginning of year	156,474
Projected benefit obligation at beginning of year	156,989	181,137
Service cost	453	699	608
Interest cost	6,885	6,327	6,832
Defined Benefit Plan, Plan Amendments	719	0
Benefit payments	(7,587)	(7,097)
Actuarial loss (gain)	(35,863)	24,077
Projected benefit obligation at end of year	193,322	156,989	181,137
Accumulated postretirement benefit obligation at end of year	192,638	156,474
Change in Plan Assets [Abstract]
Fair value of plan assets at beginning of year	168,408	187,150
Defined Benefit Plan, Actual (Loss) Return on Plan Assets	22,521	(11,966)
Employer contributions	329	321
Fair value of plan assets at end of year	183,671	168,408	187,150
Defined Benefit Plan, Amounts Recognized in Balance Sheet [Abstract]
Funded status – net prepaid (liability)	(9,651)	11,419
Defined Benefit Plan, Assets for Plan Benefits	7,092	16,515
Accrued liabilities	(322)	(325)
Pension benefit obligations	(16,421)	(4,771)
Net amount recognized	(9,651)	11,419
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	(45,009)	(25,002)
Pension and Other Postretirement Benefit Plans, Accumulated Other Comprehensive Income (Loss), Net Prior Service Cost, before Tax	(1,731)	(1,295)
Pension and Other Postretirement Benefit Plans, Accumulated Other Comprehensive Income (Loss), before Tax	$ (46,740)	$ (26,297)