Financial Instruments - Fair Value Assets and Liabilities Measured on Recurring Basis (Details) - Recurring - USD ($) $ in Thousands	Jun. 30, 2020	Dec. 31, 2019
Restricted investment:
Rabbi Trust	$ 30,081	$ 28,332
Fixed income securities	1,829	1,892
Quoted Prices in Active Markets (Level 1)
Restricted investment:
Rabbi Trust	0	0
Fixed income securities	0	0
Significant Other Observable Inputs (Level 2)
Restricted investment:
Rabbi Trust	30,081	28,332
Fixed income securities	1,829	1,892
Significant Unobservable Inputs (Level 3)
Restricted investment:
Rabbi Trust	0	0
Fixed income securities	0	0
Interest Rate Swap
Liabilities:
Interest rate swap derivatives	7,857	1,869
Interest Rate Swap | Quoted Prices in Active Markets (Level 1)
Liabilities:
Interest rate swap derivatives	0	0
Interest Rate Swap | Significant Other Observable Inputs (Level 2)
Liabilities:
Interest rate swap derivatives	7,857	1,869
Interest Rate Swap | Significant Unobservable Inputs (Level 3)
Liabilities:
Interest rate swap derivatives	$ 0	$ 0