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Income Taxes (Tables) | 12 Months Ended
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Dec. 31, 2021
Income Taxes |
Schedule of Components of Income Tax Expense (Benefit) | ​
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ | ​ | Year Ended December 31,
(in thousands) |  | 2021 |  | 2020 |  | 2019
Current tax provision (benefit): | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
U.S. federal | ​ | $ | (1) | ​ | $ | (99) | ​ | $ | 75
State | ​ |  | 366 | ​ |  | 326 | ​ |  | 377
Total current | ​ | ​ | 365 | ​ | ​ | 227 | ​ | ​ | 452
Deferred tax provision (benefit): | ​ | ​ |  | ​ | ​ |  | ​ | ​ |
U.S. federal | ​ | ​ | 8,800 | ​ | ​ | (17,246) | ​ | ​ | (35,597)
State | ​ |  | 1,579 | ​ |  | (518) | ​ |  | (4,000)
Total deferred | ​ | ​ | 10,379 | ​ | ​ | (17,764) | ​ | ​ | (39,597)
Provision for (benefit from) income taxes | ​ | $ | 10,744 | ​ | $ | (17,537) | ​ | $ | (39,145)
Schedule of Effective Income Tax Rate Reconciliation | ​
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ | ​ | Year Ended December 31,
(in thousands) | ​ | 2021 |  | 2020 |  | 2019
Income taxes at U.S. federal statutory rate |  | $ | 8,182 |  | $ | (18,056) |  | $ | 12,276
Net state income taxes | ​ |  | 1,374 | ​ |  | (817) | ​ |  | 1,634
Tax credits | ​ |  | (720) | ​ |  | (1,256) | ​ |  | (1,757)
Unrecognized tax benefits (1) | ​ |  | 598 | ​ |  | 772 | ​ |  | (1,958)
Valuation allowances and write off of tax attributes (2) | ​ |  | (167) | ​ |  | 236 | ​ |  | (50,219)
Executive compensation limitation | ​ |  | 1,559 | ​ |  | 1,159 | ​ |  | 1,102
Stock | ​ |  | 162 | ​ |  | 538 | ​ |  | 66
Other | ​ |  | (244) | ​ |  | (113) | ​ |  | (289)
Provision for (benefit from) income taxes | ​ | $ | 10,744 | ​ | $ | (17,537) | ​ | $ | (39,145)
(1) | Includes the expiration of statute of limitations and in 2019, also reflects a decrease in our uncertain tax benefit, net of federal benefit, due to settlements of tax audits. See “Unrecognized Tax Benefits” below for further details.
(2) | See “Tax Attributes and Valuation Allowances” below for further details.
Schedule of Deferred Tax Assets and Liabilities | ​
​ | ​ | ​ | ​ | ​ | ​ | ​
​ | ​ | December 31,
(in thousands) | ​ | 2021 | ​ | 2020
Deferred tax assets: |  | ​ |  |  | ​ |
Net operating loss carryforwards | ​ | $ | 196,654 | ​ | $ | 158,916
Accrued liabilities | ​ |  | 4,527 | ​ |  | 3,133
Other | ​ |  | 12,503 | ​ |  | 12,124
​ | ​ |  | 213,684 | ​ |  | 174,173
Valuation allowances (1) | ​ |  | (735) | ​ |  | (1,027)
Total deferred tax assets | ​ | ​ | 212,949 | ​ | ​ | 173,146
​ | ​ | ​ | ​ | ​ | ​ | ​
Deferred tax liabilities: | ​ |  |  | ​ |  |
Property, plant and equipment | ​ | ​ | (7,762) | ​ | ​ | (6,066)
Basis difference in the Partnership | ​ |  | (151,469) | ​ |  | (103,721)
Other | ​ |  | (6,975) | ​ |  | (7,150)
Total deferred tax liabilities | ​ |  | (166,206) | ​ |  | (116,937)
Net deferred tax asset (2) | ​ | $ | 46,743 | ​ | $ | 56,209
(1) | See “Tax Attributes and Valuation Allowances” below for further details.
(2) | The 2021 and 2020 net deferred tax assets are reflected in our consolidated balance sheets as deferred tax assets of $47.9 million and $56.9 million, respectively, and deferred tax liabilities of $1.1 million and $0.7 million, respectively.
Schedule of Tax Attributes and Valuation Allowances | ​
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ | ​ | Year Ended December 31,
(in thousands) | ​ | 2021 |  | 2020 |  | 2019
Balance at beginning of period (1) |  | $ | (1,027) |  | $ | (822) |  | $ | (45,439)
Additions to valuation allowance | ​ | ​ | - | ​ | ​ | (205) | ​ | ​ | (580)
Reductions to valuation allowance (1) | ​ | ​ | 292 | ​ | ​ | - | ​ | ​ | 45,197
Balance at end of period | ​ | $ | (735) | ​ | $ | (1,027) | ​ | $ | (822)
Schedule of Unrecognized Tax Benefits Roll Forward | ​
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ | ​ | Year Ended December 31,
(in thousands) | ​ | 2021 |  | 2020 |  | 2019
Beginning balance |  | $ | 18,892 |  | $ | 18,453 |  | $ | 19,560
Additions based on tax positions related to current year | ​ |  | 2,246 | ​ |  | 2,397 | ​ |  | 2,227
Additions based on tax positions related to prior years | ​ |  | 632 | ​ |  | — | ​ |  | 2,047
Reductions based on settlement refunds from government authorities | ​ |  | — | ​ |  | — | ​ |  | (4,414)
Reductions based on tax positions related to prior years | ​ |  | (138) | ​ |  | (73) | ​ |  | (51)
Reductions based on lapse of statute of limitations | ​ |  | (2,038) | ​ |  | (1,885) | ​ |  | (916)
Ending balance | ​ | $ | 19,594 | ​ | $ | 18,892 | ​ | $ | 18,453

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