CONSOLIDATED BALANCE SHEETS - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Current assets:
Cash and cash equivalents	$ 646,663	$ 519,101
Short-term marketable securities	64,844	51,857
Accounts receivable, net of allowances aggregating $41,807 and $44,749, respectively	703,199	611,534
Unbilled accounts receivable	69,912	55,681
Inventories and supplies	228,682	220,498
Prepaid expenses and other current assets	70,864	67,051
Total current assets	1,784,164	1,525,722
Property, plant and equipment, net	1,508,356	1,525,298
Other assets:
Operating lease right-of-use assets	137,429	150,341
Goodwill	543,028	527,023
Permits and other intangibles, net	366,497	386,620
Other	14,825	16,516
Total other assets	1,061,779	1,080,500
Total assets	4,354,299	4,131,520
Current liabilities:
Current portion of long-term debt	7,535	7,535
Accounts payable	286,565	195,878
Deferred revenue	86,589	74,066
Accrued expenses and other current liabilities	299,427	295,823
Current portion of closure, post-closure and remedial liabilities	23,288	26,093
Current portion of operating lease liabilities	36,497	36,750
Total current liabilities	739,901	636,145
Other liabilities:
Closure and post-closure liabilities, less current portion of $10,219 and $13,903, respectively	82,809	74,023
Remedial liabilities, less current portion of $13,069 and $12,190, respectively	97,747	102,623
Long-term debt, less current portion	1,546,284	1,549,641
Operating lease liabilities, less current portion	102,093	114,258
Deferred tax liabilities	231,663	230,097
Other long-term liabilities	90,242	83,182
Total other liabilities	2,150,838	2,153,824
Commitments and contingent liabilities (See Note 16)
Stockholders’ equity:
Authorized 80,000,000 shares; issued and outstanding 54,409,894 and 54,772,696 shares, respectively	544	548
Additional paid-in capital	539,747	582,749
Accumulated other comprehensive loss	(200,716)	(211,477)
Accumulated earnings	1,123,985	969,731
Total stockholders’ equity	1,463,560	1,341,551
Total liabilities and stockholders’ equity	$ 4,354,299	$ 4,131,520