CONSOLIDATED BALANCE SHEETS - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Current assets:
Cash and cash equivalents	$ 371,991	$ 226,507
Short-term marketable securities	42,421	52,856
Accounts receivable, net of allowances aggregating $38,711 and $44,315, respectively	644,738	606,952
Unbilled accounts receivable	56,326	54,794
Deferred costs	21,746	18,770
Inventories and supplies	214,744	199,479
Prepaid expenses and other current assets	48,942	42,800
Total current assets	1,400,908	1,202,158
Property, plant and equipment, net	1,588,151
Property, plant and equipment, net		1,561,978
Other assets:
Operating lease right-of-use assets	162,206
Goodwill	525,013	514,189
Permits and other intangibles, net	419,066	441,875
Other	13,560	18,121
Total other assets	1,119,845	974,185
Total assets	4,108,904	3,738,321
Current liabilities:
Current portion of long-term obligations	7,535	7,535
Accounts payable	298,375	276,461
Deferred revenue	73,370	61,843
Accrued expenses	276,540	233,405
Current portion of closure, post-closure and remedial liabilities	23,301	23,034
Current portion of operating lease liabilities	40,979
Total current liabilities	720,100	602,278
Other liabilities:
Closure and post-closure liabilities, less current portion of $7,283 and $9,592, respectively	68,368	60,339
Remedial liabilities, less current portion of $16,018 and $13,442, respectively	98,155	107,575
Long-term obligations, less current portion	1,554,116	1,565,021
Operating lease liabilities, less current portion	121,020
Deferred taxes, unrecognized tax benefits and other long-term liabilities	277,332	233,352
Total other liabilities	2,118,991	1,966,287
Commitments and contingent liabilities (See Note 18)
Stockholders' equity:
Common stock, $.01 par value: Authorized 80,000,000 shares; issued and outstanding 56,501,190 and 57,297,978 shares, respectively	558	558
Additional paid-in capital	644,412	655,415
Accumulated other comprehensive loss	(210,051)	(223,371)
Accumulated earnings	834,894	737,154
Total stockholders' equity	1,269,813	1,169,756
Total liabilities and stockholders' equity	$ 4,108,904	$ 3,738,321