CONSOLIDATED BALANCE SHEETS - USD ($) $ in Thousands	Jun. 30, 2020	Dec. 31, 2019
Current assets:
Cash and cash equivalents	$ 447,366	$ 371,991
Short-term marketable securities	59,326	42,421
Accounts receivable, net of allowances aggregating $44,632 and $38,711, respectively	572,373	644,738
Unbilled accounts receivable	44,761	56,326
Deferred costs	18,715	21,746
Inventories and supplies	219,808	214,744
Prepaid expenses and other current assets	69,455	48,942
Total current assets	1,431,804	1,400,908
Property, plant and equipment, net	1,553,808	1,588,151
Other assets:
Operating lease right-of-use assets	153,522	162,206
Goodwill	523,154	525,013
Permits and other intangibles, net	400,448	419,066
Other	14,893	13,560
Total other assets	1,092,017	1,119,845
Total assets	4,077,629	4,108,904
Current liabilities:
Current portion of long-term obligations	7,535	7,535
Accounts payable	188,340	298,375
Deferred revenue	61,902	73,370
Accrued expenses	289,414	276,540
Current portion of closure, post-closure and remedial liabilities	19,129	23,301
Current portion of operating lease liabilities	38,620	40,979
Total current liabilities	604,940	720,100
Other liabilities:
Closure and post-closure liabilities, less current portion of $5,708 and $7,283, respectively	76,933	68,368
Remedial liabilities, less current portion of $13,421 and $16,018, respectively	99,062	98,155
Long-term obligations, less current portion	1,626,871	1,554,116
Operating lease liabilities, less current portion	115,089	121,020
Deferred taxes, unrecognized tax benefits and other long-term liabilities	300,763	277,332
Total other liabilities	2,218,718	2,118,991
Commitments and contingent liabilities (See Note 16)
Stockholders’ equity:
Common stock, $0.01 par value: authorized 80,000,000 shares; issued and outstanding 55,612,860 and 55,797,734 shares, respectively	556	558
Additional paid-in capital	629,755	644,412
Accumulated other comprehensive loss	(251,829)	(210,051)
Accumulated earnings	875,489	834,894
Total stockholders’ equity	1,253,971	1,269,813
Total liabilities and stockholders’ equity	$ 4,077,629	$ 4,108,904
Common stock, par value (in USD per share)	$ 0.01	$ 0.01