FInancial Instruments (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Deferred compensation assets	$ 49,443	$ 46,145
Assets, fair value	50,201	46,215
Deferred compensation plan	40,452	37,280
Liabilities, fair value	40,452	37,772
Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Deferred compensation assets	0	0
Assets, fair value	0	0
Deferred compensation plan	0	0
Liabilities, fair value	0	0
Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Deferred compensation assets	49,443	46,145
Assets, fair value	50,201	46,215
Deferred compensation plan	40,452	37,280
Liabilities, fair value	40,452	37,772
Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Deferred compensation assets	0	0
Assets, fair value	0	0
Deferred compensation plan	0	0
Liabilities, fair value	0	0
5 year interest rate swap
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Interest rate derivative assets	196	70
5 year interest rate swap | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Interest rate derivative assets		70
10 year interest rate swap
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Interest rate derivative assets	$ 562
Interest rate derivative liabilities		492
10 year interest rate swap | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Interest rate derivative liabilities		0
10 year interest rate swap | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Interest rate derivative liabilities		492
10 year interest rate swap | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Interest rate derivative liabilities		$ 0