Significant Accounting Policies - Fair Value Measurements (Details) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2018	Dec. 31, 2017
Measurement of financial assets and liabilities at fair value on a recurring basis
Transfers among levels within the fair value hierarchy	$ 0
Carrying Value
Measurement of financial assets and liabilities at fair value on a recurring basis
Cash surrender value of life insurance policies	31,495	$ 28,645
Interest rate swap assets	12,163	651
Measured on Recurring Basis | Level 1
Measurement of financial assets and liabilities at fair value on a recurring basis
Cash surrender value of life insurance policies	31,495	28,645
Measured on Recurring Basis | Level 2
Measurement of financial assets and liabilities at fair value on a recurring basis
Interest rate swap assets	$ 12,163	$ 651