Leases (Breakdown of Lease Expense) (Details) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended
Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Leases
Operating lease cost	$ 4,592	$ 4,310	$ 13,584	$ 12,780
Amortization of leased assets	2,693	2,355	8,096	6,664
Interest on lease liabilities	196	219	622	680
Short-term lease cost	2,845	2,115	7,428	7,146
Sublease income	(15)	(110)	(45)	(406)
Lease expense	$ 10,311	$ 8,889	$ 29,685	$ 26,864

Amount of interest expense on finance lease liability.

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
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Amount of amortization expense attributable to right-of-use asset from finance lease.

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- Definition

Amount of lease cost recognized by lessee for lease contract.

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Amount of single lease cost, calculated by allocation of remaining cost of lease over remaining lease term. Includes, but is not limited to, single lease cost, after impairment of right-of-use asset, calculated by amortization of remaining right-of-use asset and accretion of lease liability.

+ References

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Amount of short-term lease cost, excluding expense for lease with term of one month or less.

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- Definition

Amount of sublease income excluding finance and operating lease expense.

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