Cash and Cash Equivalents - Summary of Cash and Cash Equivalents (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018	Mar. 31, 2018	Dec. 31, 2017
Cash and Cash Equivalents [Abstract]
Cash and due from banks	$ 6,766,158	$ 3,444,971
Securities purchased under agreements to resell	298,755	123,611
Other short-term investment securities	1,970	2,957
Total cash and cash equivalents	$ 7,066,883	$ 3,571,539	$ 2,619,384	$ 2,923,075