Employee Retirement Plans - Fair Value (Details) (Pension plan, USD $) In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Employee retirement plans
Total pension plan assets	$ 26,046	$ 24,638	$ 21,808
Level 2
Employee retirement plans
Total pension plan assets	24,730	23,412
Level 3
Employee retirement plans
Total pension plan assets	1,316	1,226	1,117
Equity holdings
Employee retirement plans
Total pension plan assets	15,936	13,582
Equity holdings | Level 2
Employee retirement plans
Total pension plan assets	15,936	13,582
Fixed-income holdings
Employee retirement plans
Total pension plan assets	8,794	9,830
Fixed-income holdings | Level 2
Employee retirement plans
Total pension plan assets	8,794	9,830
Alternative investments
Employee retirement plans
Total pension plan assets	1,316	1,226
Alternative investments | Level 3
Employee retirement plans
Total pension plan assets	$ 1,316	$ 1,226