CONSOLIDATED BALANCE SHEETS - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Current assets:
Cash and cash equivalents	$ 204,455	$ 226,507
Short-term marketable securities	55,198	52,856
Accounts receivable, net of allowances aggregating $36,398 and $44,315, respectively	632,888	606,952
Unbilled accounts receivable	52,174	54,794
Deferred costs	22,500	18,770
Inventories and supplies	203,331	199,479
Prepaid expenses and other current assets	42,640	42,800
Total current assets	1,213,186	1,202,158
Property, plant and equipment, net	1,596,917	1,561,978
Other assets:
Operating lease right-of-use asset	173,504
Goodwill	525,044	514,189
Permits and other intangibles, net	433,853	441,875
Other	12,817	18,121
Total other assets	1,145,218	974,185
Total assets	3,955,321	3,738,321
Current liabilities:
Current portion of long-term obligations	7,535	7,535
Accounts payable	253,177	276,461
Deferred revenue	75,170	61,843
Accrued expenses	224,497	233,405
Current portion of closure, post-closure and remedial liabilities	30,747	23,034
Current portion of operating lease liabilities	42,564
Total current liabilities	633,690	602,278
Other liabilities:
Closure and post-closure liabilities, less current portion of $13,491 and $9,592, respectively	62,339	60,339
Remedial liabilities, less current portion of $17,256 and $13,442, respectively	101,019	107,575
Long-term obligations, less current portion	1,562,989	1,565,021
Operating lease liabilities, less current portion	130,704
Deferred taxes, unrecognized tax benefits and other long-term liabilities	255,113	233,352
Total other liabilities	2,112,164	1,966,287
Commitments and contingent liabilities (See Note 16)
Stockholders’ equity:
Authorized 80,000,000; shares issued and outstanding 55,859,402 and 55,847,261 shares, respectively	559	558
Additional paid-in capital	648,805	655,415
Accumulated other comprehensive loss	(214,271)	(223,371)
Accumulated earnings	774,374	737,154
Total stockholders’ equity	1,209,467	1,169,756
Total liabilities and stockholders’ equity	$ 3,955,321	$ 3,738,321