Senior Secured Credit Facilities (Details) - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
Sep. 30, 2018	Dec. 31, 2017
Debt Instrument [Line Items]
Index Rate	one-month LIBOR
Commitment Amount	$ 3,610,303	$ 3,376,158
Maximum Current Availability	1,408,923	1,250,870
Balance Outstanding	2,201,380	$ 2,125,288
Senior Secured Credit Facilities
Debt Instrument [Line Items]
Commitment Amount	660,000
Maximum Current Availability	208,416
Balance Outstanding	$ 451,584
Bank of America \| Senior Secured Credit Facilities
Debt Instrument [Line Items]
Maturity Date	Sep. 29, 2020	Sep. 29, 2020
Index Rate	1 Month Libor	1 Month Libor
Weighted Average Spread	1.90%
Interest Rate	4.10%
Commitment Amount	$ 500,000	$ 250,000
Maximum Current Availability	112,560	250,000
Balance Outstanding	$ 387,440	$ 0
Bank of America \| Debt Instrument, Interest Rate at 4.1% \| Senior Secured Credit Facilities
Debt Instrument [Line Items]
Maturity Date	Sep. 29, 2020
Index Rate	1 Month Libor
Weighted Average Spread	1.90%
Interest Rate	4.10%
Commitment Amount	$ 500,000
Maximum Current Availability	112,560
Balance Outstanding	$ 387,440
Citibank \| Debt Instrument, Interest Rate at 4.5% \| Senior Secured Credit Facilities
Debt Instrument [Line Items]
Maturity Date	Jul. 12, 2020
Index Rate	1 Month Libor
Weighted Average Spread	2.30%
Interest Rate	4.50%
Commitment Amount	$ 160,000
Maximum Current Availability	95,856
Balance Outstanding	$ 64,144

Amount, before unamortized (discount) premium and debt issuance costs, of long-term debt. Includes, but is not limited to, notes payable, bonds payable, commercial loans, mortgage loans, convertible debt, subordinated debt and other types of debt.

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The reference rate for the variable rate of the debt instrument, such as LIBOR or the US Treasury rate and the maturity of the reference rate used, such as three months or six months LIBOR.

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Face (par) amount of debt instrument at time of issuance.

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Contractual interest rate for funds borrowed, under the debt agreement.

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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Date when the debt instrument is scheduled to be fully repaid, in CCYY-MM-DD format.

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Amount of unused borrowing capacity under the long-term financing arrangement that is available to the entity as of the balance sheet date.

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Weighted average interest rate of debt outstanding.

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