Long-Term Debt - Narrative (Details) $ in Millions					3 Months Ended		6 Months Ended
		Oct. 11, 2019 USD ($)	Sep. 04, 2019	Feb. 23, 2018	Jun. 30, 2020 USD ($)	Jun. 30, 2019 USD ($)	Jun. 30, 2020 USD ($)	Jun. 30, 2019 USD ($)	Dec. 31, 2019 USD ($)
Long-Term Debt (Textual) [Abstract]
Accumulated Amortization, Deferred Finance Costs					$ 18.1		$ 18.1		$ 15.7
Gain (Loss) from debt extinguishment					0.0	$ 0.3	0.0	$ 0.7
Repayments of borrowings — senior notes							0.0	$ 88.6
Line of Credit Amendment			Sep. 04, 2019	Feb. 23, 2018
Line of Credit Facility, Collateral		adding the fixed assets of the Company’s Great Falls, MT refinery
FILO Revolver
Long-Term Debt (Textual) [Abstract]
Senior secured revolving credit facility					25.0		25.0
Notes Due April 2023 at Fixed Rate of 7.75% Interest Payments
Long-Term Debt (Textual) [Abstract]
Senior Notes, Noncurrent					325.0		325.0		325.0
7.625% Notes
Long-Term Debt (Textual) [Abstract]
Senior Notes, Noncurrent	[1]				350.9		$ 350.9		351.1
Senior Notes | Maximum
Long-Term Debt (Textual) [Abstract]
Fixed charge coverage ratio							1.9
Revolving Credit Facility
Long-Term Debt (Textual) [Abstract]
Maturity date				Feb. 23, 2023
Frequency of interest payment							quarterly
Senior secured revolving credit facility					600.0		$ 600.0		600.0
Expansion Feature		$ 99.6
Incremental uncommitted expansion feature					$ 500.0		$ 500.0
Consecutive Quarters, Margin Reduction Threshold					5.5		5.5
Customary letter of credit fee, including a fronting fee per annum on the stated amount of each outstanding letter of credit							0.125%
Outstanding standby letters of credit					$ 25.3		$ 25.3		$ 42.5
Financial covenant							if the Company’s availability to borrow loans under the revolving credit facility falls below the sum of the greater of (i) 10% of the borrowing base then in effect, or 15% while the Great Falls, MT refinery is included in the borrowing base, and (ii) $35.0 million (which amount is subject to increase in proportion to revolving commitment increases), plus the amount of FILO loans outstanding, then the Company will be required to maintain as of the end of each fiscal quarter a Fixed Charge Coverage Ratio (as defined in the Credit Agreement) of at least 1.0 to 1.0.
Line of Credit Facility, Current Borrowing Capacity					$ 279.2		$ 279.2
Revolving Credit Facility | Maximum
Long-Term Debt (Textual) [Abstract]
Letter of Credit Sublimit					90.00%		90.00%		90.00%
Basis points		25.00%
Unutilized commitments fee to the lender under the revolving credit facility							0.375%
Revolving Credit Facility | Minimum
Long-Term Debt (Textual) [Abstract]
Basis points							25.00%
Unutilized commitments fee to the lender under the revolving credit facility							0.25%
London Interbank Offered Rate (LIBOR) | FILO Revolver
Long-Term Debt (Textual) [Abstract]
Basis points							250.00%
London Interbank Offered Rate (LIBOR) | Revolving Credit Facility
Long-Term Debt (Textual) [Abstract]
Basis points							150.00%
Prime Rate | FILO Revolver
Long-Term Debt (Textual) [Abstract]
Basis points							150.00%
Prime Rate | Revolving Credit Facility
Long-Term Debt (Textual) [Abstract]
Basis points							50.00%
Quarterly Average Availability Percentage, Range 1 | London Interbank Offered Rate (LIBOR) | FILO Revolver
Long-Term Debt (Textual) [Abstract]
Basis points							2.50%
Quarterly Average Availability Percentage, Range 1 | London Interbank Offered Rate (LIBOR) | Revolving Credit Facility
Long-Term Debt (Textual) [Abstract]
Basis points							1.50%
Quarterly Average Availability Percentage, Range 1 | Prime Rate | FILO Revolver
Long-Term Debt (Textual) [Abstract]
Basis points							1.50%
Quarterly Average Availability Percentage, Range 1 | Prime Rate | Revolving Credit Facility
Long-Term Debt (Textual) [Abstract]
Basis points							0.50%
Quarterly Average Availability Percentage, Range 2 | London Interbank Offered Rate (LIBOR) | FILO Revolver
Long-Term Debt (Textual) [Abstract]
Basis points							2.75%
Quarterly Average Availability Percentage, Range 2 | London Interbank Offered Rate (LIBOR) | Revolving Credit Facility
Long-Term Debt (Textual) [Abstract]
Basis points							1.75%
Quarterly Average Availability Percentage, Range 2 | Prime Rate | FILO Revolver
Long-Term Debt (Textual) [Abstract]
Basis points							1.75%
Quarterly Average Availability Percentage, Range 2 | Prime Rate | Revolving Credit Facility
Long-Term Debt (Textual) [Abstract]
Basis points							0.75%
Quarterly Average Availabiity Percentage, Range 3 | London Interbank Offered Rate (LIBOR) | FILO Revolver
Long-Term Debt (Textual) [Abstract]
Basis points							3.00%
Quarterly Average Availabiity Percentage, Range 3 | London Interbank Offered Rate (LIBOR) | Revolving Credit Facility
Long-Term Debt (Textual) [Abstract]
Basis points							2.00%
Quarterly Average Availabiity Percentage, Range 3 | Prime Rate | FILO Revolver
Long-Term Debt (Textual) [Abstract]
Basis points							2.00%
Quarterly Average Availabiity Percentage, Range 3 | Prime Rate | Revolving Credit Facility
Long-Term Debt (Textual) [Abstract]
Basis points							1.00%

[1] The balance includes a fair value interest rate hedge adjustment, which increased the debt balance by $0.9 million and $1.1 million as of June 30, 2020 and December 31, 2019, respectively.