Summary of Significant Accounting Policies - Loans Purchased and Allowance for Loan Losses (Details) - USD ($) $ in Thousands	1 Months Ended	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2017	Dec. 31, 2016
Loans Purchased
Purchase of loans receivable held for investment		$ 24,640	$ 0
Allowance for Loan Losses
Length of period over which historical loss experience		5 years
Single family
Loans Purchased
Purchase of loans receivable held for investment	$ 24,600