Summary of Critical and Significant Accounting Policies - Allowance For Credit Loss (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Jan. 01, 2020
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	$ 0
Impact of adoption ASC 326	0	$ 0
Reversal of provision for credit losses	687	0	$ 0
Ending balance	0	0
Accelerated interest income	11,800	7,000
Mortgage Backed Securities
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	0
Impact of adoption ASC 326	0	0
Reversal of provision for credit losses	(13,644)
Ending balance	$ 0	$ 0
Cumulative Effect, Period of Adoption, Adjustment | Mortgage Backed Securities
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Impact of adoption ASC 326				$ 13,644