Summary of Significant Accounting Policies - Assets and liabilities measured at fair value on recurring basis (Details) - Fair Value, Measurements, Recurring - USD ($) $ in Thousands	Sep. 30, 2016	Dec. 31, 2015
Level 1
Assets:
Total Assets	$ 764	$ 2,196
Level 1 | Money Market Deposit Escrow
Assets:
Money Market Escrow Accounts - AEV, Inc. and Foley Company Sales		2,000
Level 1 | Money Market and Mutual Funds
Assets:
Other Assets - Nonqualified Retirement Savings Plan	764	196
Level 2
Assets:
Total Assets	7,914	8,093
Liabilities:
Total Liabilities	49	199
Level 2 | Government-Backed and Government-Sponsored Enterprises' Debt Securities
Assets:
Investments - Held by Captive Insurance Company	3,506	4,235
Level 2 | Corporate Debt Securities
Assets:
Investments - Held by Captive Insurance Company	4,408	3,858
Level 2 | Forward Gasoline Purchase Contracts
Liabilities:
Derivative Liabilities	$ 49	$ 199