Cash and Cash Equivalents - Summary of Cash and Cash Equivalents (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018	Jun. 30, 2018	Dec. 31, 2017
Cash and Cash Equivalents [Abstract]
Cash and due from banks	$ 8,727,265	$ 3,444,971
Securities purchased under agreements to resell	291,678	123,611
Other short-term investment securities	1,982	2,957
Total cash and cash equivalents	$ 9,020,925	$ 3,571,539	$ 2,712,101	$ 2,923,075