Fair Value Measurements - Notional Amounts and Fair Values of Derivatives by Product (Details) - Not Designated as Hedging Instrument - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Derivatives, Fair Value [Line Items]
Derivatives asset, Notional	$ 7,970
Derivatives asset, Fair Value	15
Derivative liability, notional	3,449,249	$ 5,229,373
Derivative liability, Fair Value	220	1,414
Other Contracts \| TBAs
Derivatives, Fair Value [Line Items]
Derivatives asset, Notional	7,970
Derivatives asset, Fair Value	15
Derivative liability, notional	7,936	13,245
Derivative liability, Fair Value	3	124
Other Contracts \| Auction Rate Securities Purchase Commitment
Derivatives, Fair Value [Line Items]
Derivative liability, notional	1,313	7,128
Derivative liability, Fair Value	195	1,023
Commodity Contracts \| Futures
Derivatives, Fair Value [Line Items]
Derivative liability, notional	3,440,000	5,209,000
Derivative liability, Fair Value	$ 22	$ 267

Nominal or face amount used to calculate payments on the derivative asset.

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Fair value, after offset of derivative asset, of financial liability or contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset, elected not to be and before offset against a right to receive collateral under a master netting arrangement. Includes liabilities elected not to be offset. Excludes liabilities not subject to a master netting arrangement.

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Nominal or face amount used to calculate payments on the derivative liability.

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