Summary of Significant Accounting Policies - Cash, Cash Equivalents and Restricted Cash (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018	Sep. 30, 2018
Restricted Cash and Cash Equivalents Items [Line Items]
Cash and cash equivalents	$ 371,360	$ 161,013	$ 16,188
Total cash, cash equivalents and restricted cash	374,047	205,100	37,344
Release Account
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash		16,141	9,313
Liquidity Reserve
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash		5,599	5,570
Lender controlled accounts
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash	2,687	22,347	4,725
Other
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash	$ 0	$ 0	$ 1,548