Summary of Significant Accounting Policies Newly Adopted and Recently Issued But Not Yet Adopted ASU (Details) - USD ($) $ in Millions	Mar. 31, 2019	Dec. 31, 2018	Mar. 31, 2018	Dec. 31, 2017
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Securities held to maturity	$ 36,503	$ 36,771
Accumulated other comprehensive loss	(660)	$ (1,263)	$ (1,599)	$ (926)
Lease liabilities	1,836
Right-of-use assets	1,547
Accounting Standards Update 2016-02
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Lease liabilities	1,900
Right-of-use assets	$ 1,601