CONSOLIDATED BALANCE SHEETS - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Current assets:
Cash and cash equivalents	$ 432,205	$ 371,991
Short-term marketable securities	62,143	42,421
Accounts receivable, net of allowances aggregating $42,781 and $38,711, respectively	658,482	644,738
Unbilled accounts receivable	51,215	56,326
Deferred costs	21,270	21,746
Inventories and supplies	216,532	214,744
Prepaid expenses and other current assets	44,629	48,942
Total current assets	1,486,476	1,400,908
Property, plant and equipment, net	1,547,119	1,588,151
Other assets:
Operating lease right-of-use assets	160,526	162,206
Goodwill	519,627	525,013
Permits and other intangibles, net	406,881	419,066
Other	11,392	13,560
Total other assets	1,098,426	1,119,845
Total assets	4,132,021	4,108,904
Current liabilities:
Current portion of long-term obligations	7,535	7,535
Accounts payable	267,892	298,375
Deferred revenue	71,243	73,370
Accrued expenses	255,513	276,540
Current portion of closure, post-closure and remedial liabilities	16,231	23,301
Current portion of operating lease liabilities	39,998	40,979
Total current liabilities	658,412	720,100
Other liabilities:
Closure and post-closure liabilities, less current portion of $4,333 and $7,283, respectively	76,106	68,368
Remedial liabilities, less current portion of $11,898 and $16,018, respectively	98,966	98,155
Long-term obligations, less current portion	1,702,992	1,554,116
Operating lease liabilities, less current portion	120,649	121,020
Deferred taxes, unrecognized tax benefits and other long-term liabilities	269,091	277,332
Total other liabilities	2,267,804	2,118,991
Commitments and contingent liabilities (See Note 16)
Stockholders’ equity:
Authorized 80,000,000 shares; issued and outstanding 55,554,925 and 55,797,734 shares, respectively	556	558
Additional paid-in capital	628,140	644,412
Accumulated other comprehensive loss	(269,357)	(210,051)
Accumulated earnings	846,466	834,894
Total stockholders’ equity	1,205,805	1,269,813
Total liabilities and stockholders’ equity	$ 4,132,021	$ 4,108,904