Investment Securities - Additional Information (Details) $ in Millions	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Mar. 31, 2018 USD ($)
Debt Securities, Available-for-sale [Line Items]
Securities available for sale	$ 46,150	$ 37,655
Securities held to maturity	$ 36,771	28,984
Debt Securities, Available-for-sale, Unrealized Loss Position, Number of Positions	1,390
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss	$ 972	458
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	$ 675	373
Held-to-maturity, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions	360
Debt Securities, Held-to-maturity, Unrealized Loss Position, Accumulated Loss	$ 466
Other than Temporary Impairment, Credit Losses Recognized in Earnings, Credit Losses on Debt Securities Held	140	147
Derivative, Collateral, Obligation to Return Securities	$ 1	$ 1
Investment Securities Portfolio [Member] \| US Treasury and Government
Debt Securities, Available-for-sale [Line Items]
Percentage of portfolio	96.00%	95.00%
Accounting Standards Update 2017-12
Debt Securities, Available-for-sale [Line Items]
Securities held to maturity	$ 9,000		$ 9,000
AOCI associated with transferred held to maturity securities, pretax			535
AOCI associated with transferred held to maturity securities, after tax			$ 407
Collateral Pledged [Member]
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Held-to-maturity, Restricted	$ 16,300	$ 8,500

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AOCI associated with transferred held to maturity securities, pretax

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Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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Amount of accumulated unrealized loss on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in continuous unrealized loss position for 12 months or longer.

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Number of investments in debt securities measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in unrealized loss position.

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Amount of obligation to return securities collateral under master netting arrangements that have not been offset against derivative assets.

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Amount of investment in debt security measured at amortized cost (held-to-maturity).

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Disclosure regarding factors used to determine that the impairment of securities categorized as held-to-maturity where cost exceeds fair value is not an other than temporary impairment (OTTI). This item contains disclosure of the number of investment positions in the held-to-maturity investments determined to be temporarily impaired.

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Amount of restricted investment in debt security measured at amortized cost (held-to-maturity).

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The cumulative amount of credit losses recognized in earnings related to debt securities held for which a portion of an other than temporary impairment (OTTI) was recognized in other comprehensive income (a component of shareholders' equity).

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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