Consolidated Balance Sheets - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Assets
Property, plant and equipment, at cost	$ 12,293,056	$ 11,620,019
Less: accumulated depreciation	2,336,287	2,107,142
Net property, plant and equipment	9,956,769	9,512,877
Current assets:
Cash and cash equivalents	9,736	4,827
Accounts receivable, net	108,210	154,775
Unbilled revenues	63,479	118,538
Inventory - materials and supplies	35,023	21,669
Inventory - gas stored	78,787	36,732
Prepayments and other current assets	29,871	38,594
Regulatory assets	27,398	5,085
Total current assets	352,504	380,220
Regulatory assets	1,404,837	1,362,788
Deferred charges and other assets, net	118,910	56,002
Funds restricted for construction activity	1,288	1,268
Goodwill	2,340,836	2,324,547
Operating lease right-of-use assets	55,386	60,334
Intangible assets	5,955	7,241
Total assets	14,236,485	13,705,277
Stockholders' equity:
Common stock at $0.50 par value, authorized 600,000,000 shares, issued 255,981,932 and 248,571,355 as of September 30, 2021 and December 31, 2020	127,990	124,285
Capital in excess of par value	3,698,095	3,379,057
Retained earnings	1,385,631	1,261,862
Treasury stock, at cost, 3,240,349 and 3,180,887 shares as of September 30, 2021 and December 31, 2020	(83,854)	(81,327)
Total stockholders' equity	5,127,862	4,683,877
Long-term debt, excluding current portion	5,638,961	5,545,890
Less: debt issuance costs	40,247	38,146
Long-term debt, excluding current portion, net of debt issuance costs	5,598,714	5,507,744
Commitments and contingencies (See Note 14)
Current liabilities:
Current portion of long-term debt	63,418	84,353
Loans payable	46,582	78,198
Accounts payable	163,836	177,489
Book overdraft	20,749	44,003
Accrued interest	70,461	39,408
Accrued taxes	37,502	37,172
Regulatory liabilities	171	19,866
Other accrued liabilities	113,310	123,384
Total current liabilities	516,029	603,873
Deferred credits and other liabilities:
Deferred income taxes and investment tax credits	1,352,425	1,258,098
Customers' advances for construction	110,939	99,014
Regulatory liabilities	778,416	773,310
Asset retirement obligations	1,363	1,336
Operating lease liabilities	50,086	55,642
Pension and other postretirement benefit liabilities	61,690	91,896
Other	50,856	56,713
Total deferred credits and other liabilities	2,405,775	2,336,009
Contributions in aid of construction	588,105	573,774
Total liabilities and equity	$ 14,236,485	$ 13,705,277