CONSOLIDATED BALANCE SHEETS - USD ($) $ in Thousands	Jun. 30, 2021	Dec. 31, 2020
Current assets:
Cash and cash equivalents	$ 595,574	$ 519,101
Short-term marketable securities	70,683	51,857
Accounts receivable, net of allowances aggregating $40,009 and $44,749, respectively	659,364	611,534
Unbilled accounts receivable	59,446	55,681
Inventories and supplies	215,725	220,498
Prepaid expenses and other current assets	76,524	67,051
Total current assets	1,677,316	1,525,722
Property, plant and equipment, net	1,531,289	1,525,298
Other assets:
Operating lease right-of-use assets	135,363	150,341
Goodwill	544,639	527,023
Permits and other intangibles, net	374,230	386,620
Other	13,042	16,516
Total other assets	1,067,274	1,080,500
Total assets	4,275,879	4,131,520
Current liabilities:
Current portion of long-term debt	7,535	7,535
Accounts payable	249,206	195,878
Deferred revenue	83,733	74,066
Accrued expenses	311,656	295,823
Current portion of closure, post-closure and remedial liabilities	23,865	26,093
Current portion of operating lease liabilities	35,074	36,750
Total current liabilities	711,069	636,145
Other liabilities:
Closure and post-closure liabilities, less current portion of $9,752 and $13,903, respectively	83,742	74,023
Remedial liabilities, less current portion of $14,113 and $12,190, respectively	98,341	102,623
Long-term debt, less current portion	1,547,398	1,549,641
Operating lease liabilities, less current portion	101,377	114,258
Deferred tax liabilities	228,718	230,097
Other long-term liabilities	95,647	83,182
Total other liabilities	2,155,223	2,153,824
Commitments and contingent liabilities (See Note 16)
Stockholders’ equity:
Common stock, $0.01 par value:	544	548
Additional paid-in capital	539,390	582,749
Accumulated other comprehensive loss	(188,889)	(211,477)
Accumulated earnings	1,058,542	969,731
Total stockholders’ equity	1,409,587	1,341,551
Total liabilities and stockholders’ equity	$ 4,275,879	$ 4,131,520