Cash and Cash Equivalents - Summary of Cash and Cash Equivalents (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018	Sep. 30, 2018	Dec. 31, 2017
Cash and Cash Equivalents [Abstract]
Cash and due from banks	$ 6,557,085	$ 3,444,971
Securities purchased under agreements to resell	387,119	123,611
Other short-term investment securities	1,992	2,957
Total cash and cash equivalents	$ 6,946,196	$ 3,571,539	$ 3,819,141	$ 2,923,075