CONSOLIDATED BALANCE SHEETS - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Current assets:
Cash and cash equivalents	$ 496,383	$ 519,101
Short-term marketable securities	74,320	51,857
Accounts receivable, net of allowances aggregating $44,407 and $44,749, respectively	620,184	611,534
Unbilled accounts receivable	55,239	55,681
Inventories and supplies	219,499	220,498
Prepaid expenses and other current assets	76,726	67,051
Total current assets	1,542,351	1,525,722
Property, plant and equipment, net	1,527,944	1,525,298
Other assets:
Operating lease right-of-use assets	142,006	150,341
Goodwill	543,605	527,023
Permits and other intangibles, net	380,053	386,620
Other	16,580	16,516
Total other assets	1,082,244	1,080,500
Total assets	4,152,539	4,131,520
Current liabilities:
Current portion of long-term debt	7,535	7,535
Accounts payable	213,355	195,878
Deferred revenue	83,165	74,066
Accrued expenses	284,212	295,823
Current portion of closure, post-closure and remedial liabilities	26,896	26,093
Current portion of operating lease liabilities	35,390	36,750
Total current liabilities	650,553	636,145
Other liabilities:
Closure and post-closure liabilities, less current portion of $11,887 and $13,903, respectively	79,218	74,023
Remedial liabilities, less current portion of $15,009 and $12,190, respectively	99,239	102,623
Long-term debt, less current portion	1,548,517	1,549,641
Operating lease liabilities, less current portion	107,554	114,258
Deferred tax liabilities	230,236	230,097
Other long-term liabilities	88,772	83,182
Total other liabilities	2,153,536	2,153,824
Commitments and contingent liabilities (See Note 16)
Stockholders’ equity:
Common stock, $0.01 par value: authorized 80,000,000 shares; issued and outstanding 54,550,817 and 54,772,696 shares, respectively	546	548
Additional paid-in capital	555,966	582,749
Accumulated other comprehensive loss	(199,529)	(211,477)
Accumulated earnings	991,467	969,731
Total stockholders’ equity	1,348,450	1,341,551
Total liabilities and stockholders’ equity	$ 4,152,539	$ 4,131,520