CONSOLIDATED BALANCE SHEETS - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Current assets:
Cash and cash equivalents	$ 215,497	$ 319,399
Short-term marketable securities	37,380	38,179
Accounts receivable, net of allowances aggregating $32,260 and $27,799, respectively	608,645	528,924
Unbilled accounts receivable	63,964	35,922
Deferred costs	19,849	20,445
Inventories and supplies	196,045	176,012
Prepaid expenses and other current assets	35,441	35,175
Total current assets	1,176,821	1,154,056
Property, plant and equipment, net	1,614,429	1,587,365
Other assets:
Goodwill	514,102	478,523
Permits and other intangibles, net	451,355	469,128
Other	17,622	17,498
Total other assets	983,079	965,149
Total assets	3,774,329	3,706,570
Current liabilities:
Current portion of long-term obligations	7,535	4,000
Accounts payable	248,405	224,231
Deferred revenue	65,172	67,822
Accrued expenses	229,932	187,982
Current portion of closure, post-closure and remedial liabilities	25,256	19,782
Total current liabilities	576,300	503,817
Other liabilities:
Closure and post-closure liabilities, less current portion of $9,576 and $6,444, respectively	57,805	54,593
Remedial liabilities, less current portion of $15,680 and $13,338, respectively	105,032	111,130
Long-term obligations, less current portion	1,616,156	1,625,537
Deferred taxes, unrecognized tax benefits and other long-term liabilities	221,712	223,291
Total other liabilities	2,000,705	2,014,551
Commitments and contingent liabilities (See Note 16)
Stockholders’ equity:
Authorized 80,000,000; shares issued and outstanding 55,997,893 and 56,501,190 shares, respectively	560	565
Additional paid-in capital	661,546	686,962
Accumulated other comprehensive loss	(185,505)	(172,407)
Accumulated earnings	720,723	673,082
Total stockholders’ equity	1,197,324	1,188,202
Total liabilities and stockholders’ equity	$ 3,774,329	$ 3,706,570