CONSOLIDATED BALANCE SHEETS - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Current assets:
Cash and cash equivalents	$ 475,706	$ 371,991
Short-term marketable securities	56,639	42,421
Accounts receivable, net of allowances aggregating $41,714 and $38,711, respectively	602,069	644,738
Unbilled accounts receivable	59,438	56,326
Deferred costs	20,212	21,746
Inventories and supplies	220,884	214,744
Prepaid expenses and other current assets	58,711	48,942
Total current assets	1,493,659	1,400,908
Property, plant and equipment, net	1,539,333	1,588,151
Other assets:
Operating lease right-of-use assets	146,454	162,206
Goodwill	524,261	525,013
Permits and other intangibles, net	392,401	419,066
Other	10,079	13,560
Total other assets	1,073,195	1,119,845
Total assets	4,106,187	4,108,904
Current liabilities:
Current portion of long-term obligations	7,535	7,535
Accounts payable	213,776	298,375
Deferred revenue	67,412	73,370
Accrued expenses	293,200	276,540
Current portion of closure, post-closure and remedial liabilities	22,324	23,301
Current portion of operating lease liabilities	36,814	40,979
Total current liabilities	641,061	720,100
Other liabilities:
Closure and post-closure liabilities, less current portion of $7,146 and $7,283, respectively	77,070	68,368
Remedial liabilities, less current portion of $15,178 and $16,018, respectively	100,389	98,155
Long-term obligations, less current portion	1,550,756	1,554,116
Operating lease liabilities, less current portion	110,097	121,020
Deferred taxes, unrecognized tax benefits and other long-term liabilities	322,099	277,332
Total other liabilities	2,160,411	2,118,991
Commitments and contingent liabilities (See Note 16)
Stockholders’ equity:
Common stock, $0.01 par value: authorized 80,000,000 shares; issued and outstanding 55,245,611 and 55,797,734 shares, respectively	552	558
Additional paid-in capital	613,208	644,412
Accumulated other comprehensive loss	(239,444)	(210,051)
Accumulated earnings	930,399	834,894
Total stockholders’ equity	1,304,715	1,269,813
Total liabilities and stockholders’ equity	$ 4,106,187	$ 4,108,904
Common stock, par value (in USD per share)	$ 0.01	$ 0.01
Common stock, issued shares (in shares)	55,245,611	55,797,734