CONSOLIDATED BALANCE SHEETS - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Current assets:
Cash and cash equivalents	$ 282,233	$ 226,507
Short-term marketable securities	46,877	52,856
Accounts receivable, net of allowances aggregating $37,998 and $44,315, respectively	641,667	606,952
Unbilled accounts receivable	58,842	54,794
Deferred costs	21,939	18,770
Inventories and supplies	210,827	199,479
Prepaid expenses and other current assets	38,199	42,800
Total current assets	1,300,584	1,202,158
Property, plant and equipment, net	1,593,993	1,561,978
Other assets:
Operating lease right-of-use assets	164,302
Goodwill	524,581	514,189
Permits and other intangibles, net	425,863	441,875
Other	12,539	18,121
Total other assets	1,127,285	974,185
Total assets	4,021,862	3,738,321
Current liabilities:
Current portion of long-term obligations	7,535	7,535
Accounts payable	277,545	276,461
Deferred revenue	73,157	61,843
Accrued expenses	253,455	233,405
Current portion of closure, post-closure and remedial liabilities	26,986	23,034
Current portion of operating lease liabilities	41,364
Total current liabilities	680,042	602,278
Other liabilities:
Closure and post-closure liabilities, less current portion of $9,896 and $9,592, respectively	64,263	60,339
Remedial liabilities, less current portion of $17,090 and $13,442, respectively	100,179	107,575
Long-term obligations, less current portion	1,555,257	1,565,021
Operating lease liabilities, less current portion	122,668
Deferred taxes, unrecognized tax benefits and other long-term liabilities	263,658	233,352
Total other liabilities	2,106,025	1,966,287
Commitments and contingent liabilities (See Note 16)
Stockholders’ equity:
Authorized 80,000,000 shares; issued and outstanding 55,807,524 and 55,847,261 shares, respectively	558	558
Additional paid-in capital	648,184	655,415
Accumulated other comprehensive loss	(223,690)	(223,371)
Accumulated earnings	810,743	737,154
Total stockholders’ equity	1,235,795	1,169,756
Total liabilities and stockholders’ equity	$ 4,021,862	$ 3,738,321