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Fair Value Measurement (Fair Value Option) (Details) - USD ($)
$ in Thousands
Dec. 31, 2020
Dec. 31, 2019
Fair Value Disclosures [Abstract]    
Fair Value $ 688,079 $ 513,431
Aggregate unpaid principal balance 654,555 496,683
Fair value less aggregate unpaid principal balance $ 33,524 $ 16,748