CONSOLIDATED BALANCE SHEETS - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Current assets:
Cash and cash equivalents	$ 519,101	$ 371,991
Short-term marketable securities	51,857	42,421
Accounts receivable, net of allowances aggregating $44,749 and $38,711, respectively	611,534	644,738
Unbilled accounts receivable	55,681	56,326
Inventories and supplies	220,498	214,744
Prepaid expenses and other current assets	67,051	70,688
Total current assets	1,525,722	1,400,908
Property, plant and equipment, net	1,525,298	1,588,151
Other assets:
Operating lease right-of-use assets	150,341	162,206
Goodwill	527,023	525,013
Permits and other intangibles, net	386,620	419,066
Other	16,516	13,560
Total other assets	1,080,500	1,119,845
Total assets	4,131,520	4,108,904
Current liabilities:
Current portion of long-term debt	7,535	7,535
Accounts payable	195,878	298,375
Deferred revenue	74,066	73,370
Accrued expenses	295,823	276,540
Current portion of closure, post-closure and remedial liabilities	26,093	23,301
Current portion of operating lease liabilities	36,750	40,979
Total current liabilities	636,145	720,100
Other liabilities:
Closure and post-closure liabilities, less current portion of $13,903 and $7,283, respectively	74,023	68,368
Remedial liabilities, less current portion of $12,190 and $16,018, respectively	102,623	98,155
Long-term debt, less current portion	1,549,641	1,554,116
Operating lease liabilities, less current portion	114,258	121,020
Deferred tax liabilities	230,097	231,337
Other long-term liabilities	83,182	45,995
Total other liabilities	2,153,824	2,118,991
Commitments and contingent liabilities (See Note 17)
Stockholders' equity:
Common stock, $.01 par value: Authorized 80,000,000 shares; issued and outstanding — and 55,797,734 shares, respectively	548	558
Additional paid-in capital	582,749	644,412
Accumulated other comprehensive loss	(211,477)	(210,051)
Accumulated earnings	969,731	834,894
Total stockholders' equity	1,341,551	1,269,813
Total liabilities and stockholders' equity	$ 4,131,520	$ 4,108,904