Summary of Significant Accounting Policies - Restricted Cash (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Cash and Cash Equivalents [Line Items]
Restricted cash	$ 22,905	$ 27,662	$ 45,018	$ 59,767
Lender Reserves [Member]
Cash and Cash Equivalents [Line Items]
Restricted cash	21,500	26,400
Restricted Lockbox Accounts [Member]
Cash and Cash Equivalents [Line Items]
Restricted cash	$ 1,400	$ 1,300