Summary of Significant Accounting Policies - Equity Securities (Details) - USD ($) $ in Thousands	Jul. 01, 2019	Dec. 31, 2018	Aug. 03, 2017
Related Party Transaction [Line Items]
Sale of stock, consideration received on transaction			$ 493,866
Class A Common Shares | RMR Inc
Related Party Transaction [Line Items]
Common shares owned in RMR Inc. (in shares)		2,801,060
Shares acquired (in shares)		1,586,836
IPO | RMR Inc
Related Party Transaction [Line Items]
Sale of stock, consideration received on transaction	$ 104,674