Employee Retirement Plans (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Amounts recognized in the consolidated balance sheets consisted
Retiree health benefit obligation	$ 6,774	$ 4,654
Pension plan
Change in projected benefit obligation:
Benefit obligation at beginning of year	31,715	36,209
Service cost	216	246	268
Interest cost	1,496	1,449	1,482
Actuarial (gain) loss	6,156	(5,031)
Benefits paid	(1,221)	(1,158)
Benefit obligation at end of year	38,362	31,715	36,209
Change in plan assets:
Fair value of plan assets at beginning of year	24,638	21,808
Actual return on plan assets	1,221	3,160
Employer contributions through December 31	1,408	828
Benefits paid	(1,221)	(1,158)
Fair value of plan assets at end of year	26,046	24,638	21,808
Defined Benefit Plan, Funded Status of Plan, Total	(12,316)	(7,077)
Components of net periodic cost:
Expected return on plan assets	(1,631)	(1,409)	(1,274)
Amortization of net (gain) loss	203	1,205	770
Net periodic cost	284	1,491	1,246
Salaried Pension Plan
Accumulated benefit obligation	38,226	31,623
Other postretirement benefit cost
Change in projected benefit obligation:
Benefit obligation at beginning of year	4,874	6,812
Service cost	159	250	281
Interest cost	213	245	360
Participant contributions	63	100
Actuarial (gain) loss	1,895	(2,085)
Benefits paid	(160)	(448)
Benefit obligation at end of year	7,044	4,874	6,812
Change in plan assets:
Benefits paid	(160)	(448)
Components of net periodic cost:
Amortization of net (gain) loss	(398)	(172)	(17)
Net periodic cost	(26)	323	624
Period of coverage under the plan	10 years
Amounts recognized in the consolidated balance sheets consisted
Accrued expenses and other current liabilities	270	220
Retiree health benefit obligation	6,774	4,654
Amounts recognized in the consolidated balance sheets	$ 7,044	$ 4,874
Salaried Pension Plan
Period of service of employees	12 years