Fair Value of Financial Instruments - Fair value, assets and liabilities measured on recurring basis (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Fair value, assets and liabilities measured on recurring and nonrecurring basis [Line Items]
Repurchase and loan agreements	$ 1,620,968	$ 1,644,230
Level 1, Quoted Prices in Active Markets
Fair value, assets and liabilities measured on recurring and nonrecurring basis [Line Items]
Repurchase and loan agreements	0	0
Level 2, Observable Inputs Other Than Level 1 Prices
Fair value, assets and liabilities measured on recurring and nonrecurring basis [Line Items]
Repurchase and loan agreements	1,630,102	1,653,720
Level 3, Unobservable Inputs
Fair value, assets and liabilities measured on recurring and nonrecurring basis [Line Items]
Repurchase and loan agreements	0	0
Fair value measurements, recurring
Fair value, assets and liabilities measured on recurring and nonrecurring basis [Line Items]
Interest rate cap derivative	114	2,070
Fair value measurements, recurring | Level 1, Quoted Prices in Active Markets
Fair value, assets and liabilities measured on recurring and nonrecurring basis [Line Items]
Interest rate cap derivative	0	0
Fair value measurements, recurring | Level 2, Observable Inputs Other Than Level 1 Prices
Fair value, assets and liabilities measured on recurring and nonrecurring basis [Line Items]
Interest rate cap derivative	114	2,070
Fair value measurements, recurring | Level 3, Unobservable Inputs
Fair value, assets and liabilities measured on recurring and nonrecurring basis [Line Items]
Interest rate cap derivative	$ 0	$ 0