Derivative Financial Instruments and Fair Value Measurements - SERP Assets Measured at Fair Value on Recurring Basis (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Money market
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
SERP assets measured at fair value on a recurring basis	$ 19	$ 22
Mutual funds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
SERP assets measured at fair value on a recurring basis	1,754	1,785
Level 1 | Money market
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
SERP assets measured at fair value on a recurring basis	0	0
Level 1 | Mutual funds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
SERP assets measured at fair value on a recurring basis	0	0
Level 2 | Money market
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
SERP assets measured at fair value on a recurring basis	19	22
Level 2 | Mutual funds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
SERP assets measured at fair value on a recurring basis	1,754	1,785
Level 3 | Money market
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
SERP assets measured at fair value on a recurring basis	0	0
Level 3 | Mutual funds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
SERP assets measured at fair value on a recurring basis	$ 0	$ 0