Available-for-Sale Securities (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Debt Securities, Available-for-sale [Line Items]
Available-for-sale securities, debt and equity securities, amortized cost basis		$ 20,847
Available-for-sale securities, debt and equity securities, accumulated gross unrealized gain, before tax		96
Available-for-sale securities, debt and equity securities, accumulated gross unrealized loss, before tax		(264)
Available-for-sale securities, debt and equity securities, fair value	$ 23,153	20,679
Available-for-sale debt securities, amortized cost basis	23,126	20,840
Available-for-sale debt securities, accumulated gross unrealized gain, before tax	341	96
Available-for-sale debt securities, accumulated gross unrealized loss, before tax	(314)	(264)
Available-for-sale debt securities, fair value	23,153	20,672
Available-for-sale equity securities, amortized cost basis		7
Available-for-sale equity securities, accumulated gross unrealized gain, before tax		0
Available-for-sale equity securities, accumulated gross unrealized loss, before tax		0
Available-for-sale equity securities, fair value		7
Transfer of held-to-maturity securities to available-for-sale securities	4,672	0	$ 0
Other comprehensive income, transfers from held-to-maturity to available-for-sale securities, before tax	7	0	0
Transfer of available-for-sale securities to held-to-maturity	1,161	0	$ 492
Agency mortgage-backed securities [Member]
Debt Securities, Available-for-sale [Line Items]
Available-for-sale debt securities, amortized cost basis	22,140	19,395
Available-for-sale debt securities, accumulated gross unrealized gain, before tax	327	47
Available-for-sale debt securities, accumulated gross unrealized loss, before tax	(305)	(247)
Available-for-sale debt securities, fair value	22,162	19,195
Agency Debentures [Member]
Debt Securities, Available-for-sale [Line Items]
Available-for-sale debt securities, amortized cost basis	833	939
Available-for-sale debt securities, accumulated gross unrealized gain, before tax	13	39
Available-for-sale debt securities, accumulated gross unrealized loss, before tax	(7)	(12)
Available-for-sale debt securities, fair value	839	966
US Treasury Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Available-for-sale debt securities, amortized cost basis		452
Available-for-sale debt securities, accumulated gross unrealized gain, before tax		10
Available-for-sale debt securities, accumulated gross unrealized loss, before tax		(4)
Available-for-sale debt securities, fair value		458
Agency Debt Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Available-for-sale debt securities, amortized cost basis	140	34
Available-for-sale debt securities, accumulated gross unrealized gain, before tax	1	0
Available-for-sale debt securities, accumulated gross unrealized loss, before tax	(2)	(1)
Available-for-sale debt securities, fair value	139	33
Municipal Bonds [Member]
Debt Securities, Available-for-sale [Line Items]
Available-for-sale debt securities, amortized cost basis	12	20
Available-for-sale debt securities, accumulated gross unrealized gain, before tax	0	0
Available-for-sale debt securities, accumulated gross unrealized loss, before tax	0	0
Available-for-sale debt securities, fair value	12	$ 20
Other [Member]
Debt Securities, Available-for-sale [Line Items]
Available-for-sale debt securities, amortized cost basis	1
Available-for-sale debt securities, accumulated gross unrealized gain, before tax	0
Available-for-sale debt securities, accumulated gross unrealized loss, before tax	0
Available-for-sale debt securities, fair value	$ 1

Fair value of available-for-sale securities transferred to held-to-maturity during the period.

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Amount, before tax, of unrealized gain in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

+ References

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-Publisher FASB
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Amount, before tax, of unrealized loss in accumulated other comprehensive income (AOCI) on 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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Amortized cost 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 before tax of unrealized gain in accumulated other comprehensive income (AOCI) on investments in equity securities classified as available-for-sale.

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Amount before tax of unrealized loss in accumulated other comprehensive income (AOCI) on investments in equity securities classified as available-for-sale.

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This item represents the cost of equity securities, which are categorized neither as held-to-maturity nor trading, net of adjustments for other-than-temporary impairments recognized in earnings (less any cumulative-effect adjustments recognized, as defined), and fair value hedge accounting adjustments, if any.

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Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.

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Amount before tax of unrealized gain in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale.

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Amount before tax of unrealized loss in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale.

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No definition available.

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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 equity securities categorized neither as held-to-maturity nor as trading.

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Amount of investment in debt security, measured at amortized cost, transferred to investment in debt security, measured at fair value, with change in fair value recognized in other comprehensive income (available-for-sale) and net income (trading).

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Amount before tax and reclassification adjustments of unrealized holding gain (loss) related to transfers of securities into the available-for-sale classification and out of the held-to-maturity classification.

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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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