CONSOLIDATED BALANCE SHEETS - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Current assets:
Cash and cash equivalents	$ 197,068	$ 319,399
Short-term marketable securities	36,862	38,179
Accounts receivable, net of allowances aggregating $34,033 and $27,799, respectively	590,580	528,924
Unbilled accounts receivable	62,762	35,922
Deferred costs	20,832	20,445
Inventories and supplies	193,544	176,012
Prepaid expenses and other current assets	34,834	35,175
Total current assets	1,136,482	1,154,056
Property, plant and equipment, net	1,609,382	1,587,365
Other assets:
Goodwill	497,251	478,523
Permits and other intangibles, net	455,920	469,128
Other	16,426	17,498
Total other assets	969,597	965,149
Total assets	3,715,461	3,706,570
Current liabilities:
Current portion of long-term obligations	4,000	4,000
Accounts payable	247,821	224,231
Deferred revenue	68,705	67,822
Accrued expenses	200,135	187,982
Current portion of closure, post-closure and remedial liabilities	23,007	19,782
Total current liabilities	543,668	503,817
Other liabilities:
Closure and post-closure liabilities, less current portion of $6,514 and $6,444, respectively	58,990	54,593
Remedial liabilities, less current portion of $16,493 and $13,338, respectively	104,782	111,130
Long-term obligations, less current portion	1,624,727	1,625,537
Deferred taxes, unrecognized tax benefits and other long-term liabilities	222,246	223,291
Total other liabilities	2,010,745	2,014,551
Commitments and contingent liabilities (See Note 16)
Stockholders’ equity:
Authorized 80,000,000; shares issued and outstanding 56,087,256 and 56,501,190 shares, respectively	561	565
Additional paid-in capital	664,948	686,962
Accumulated other comprehensive loss	(194,095)	(172,407)
Accumulated earnings	689,634	673,082
Total stockholders’ equity	1,161,048	1,188,202
Total liabilities and stockholders’ equity	$ 3,715,461	$ 3,706,570