Estimated Fair Values (Estimated Fair Value Measurements on a Recurring Basis) (Details) - USD ($) $ in Millions		Jun. 30, 2019	Dec. 31, 2018
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Trading Securities		$ 1,556	$ 55
Assets:
Available-for-sale Securities		777	865
Derivative assets		368	314
Derivative Asset, Fair Value, Gross Liability and Obligation to Return Cash, Offset	[1],[2]	301	278
Liabilities:
Derivative liabilities		9	17
Derivative Liability, Fair Value, Gross Asset and Right to Reclaim Cash, Offset	[1],[2]	(88)	(146)
US Treasury Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Trading Securities		1,498	0
Government-Sponsored Enterprises Debt Obligations [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Trading Securities		58	55
Mortgage-backed Securities, Private-label residential [Member]
Assets:
Available-for-sale Securities		777	865
Fair Value, Inputs, Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Trading Securities		0	0
Assets:
Available-for-sale Securities		0	0
Derivative assets		0	0
Grantor trust assets (included in Other assets)		52	52
Liabilities:
Derivative liabilities		0	0
Fair Value, Inputs, Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Trading Securities		1,556	55
Assets:
Available-for-sale Securities		0	0
Derivative assets		67	36
Grantor trust assets (included in Other assets)		0	0
Liabilities:
Derivative liabilities		97	163
Fair Value, Inputs, Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Trading Securities		0	0
Assets:
Available-for-sale Securities		777	865
Derivative assets		0	0
Grantor trust assets (included in Other assets)		0	0
Liabilities:
Derivative liabilities		0	0
Fair Value, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Trading Securities		0	0
Assets:
Available-for-sale Securities		0	0
Derivative, Collateral, Right to Reclaim Cash		301	278	[2]
Grantor trust assets (included in Other assets)		0	0
Liabilities:
Derivative Liability, Fair Value, Gross Asset and Right to Reclaim Cash, Offset	[2]	(88)	(146)
Fair Value, Recurring [Member] | US Treasury Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Trading Securities		0
Fair Value, Recurring [Member] | Government-Sponsored Enterprises Debt Obligations [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Trading Securities		0	0
Fair Value, Recurring [Member] | Mortgage-backed Securities, Private-label residential [Member]
Assets:
Available-for-sale Securities		0	0
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Trading Securities		0
Assets:
Grantor trust assets (included in Other assets)		52	52
Total recurring assets at fair value		52	52
Liabilities:
Total recurring liabilities at fair value		0	0
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 1 [Member] | US Treasury Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Trading Securities		0
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 1 [Member] | Government-Sponsored Enterprises Debt Obligations [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Trading Securities		0	0
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 1 [Member] | Mortgage-backed Securities, Private-label residential [Member]
Assets:
Available-for-sale Securities		0	0
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Trading Securities		1,556
Assets:
Grantor trust assets (included in Other assets)		0	0
Total recurring assets at fair value		1,623	91
Liabilities:
Total recurring liabilities at fair value		97	163
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 2 [Member] | US Treasury Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Trading Securities		1,498
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 2 [Member] | Government-Sponsored Enterprises Debt Obligations [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Trading Securities		58	55
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 2 [Member] | Mortgage-backed Securities, Private-label residential [Member]
Assets:
Available-for-sale Securities		0	0
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Trading Securities		0
Assets:
Grantor trust assets (included in Other assets)		0	0
Total recurring assets at fair value		777	865
Liabilities:
Total recurring liabilities at fair value		0	0
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 3 [Member] | Government-Sponsored Enterprises Debt Obligations [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Trading Securities		0	0
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 3 [Member] | Mortgage-backed Securities, Private-label residential [Member]
Assets:
Available-for-sale Securities		777	865
Interest rate swaps [Member] | Fair Value, Recurring [Member]
Assets:
Derivative Asset, Fair Value, Gross Liability and Obligation to Return Cash, Offset		301	278
Liabilities:
Derivative Liability, Fair Value, Gross Asset and Right to Reclaim Cash, Offset		(88)	(146)
Interest rate swaps [Member] | Fair Value, Recurring [Member] | Fair Value, Inputs, Level 1 [Member]
Assets:
Derivative assets		0	0
Liabilities:
Derivative liabilities		0	0
Interest rate swaps [Member] | Fair Value, Recurring [Member] | Fair Value, Inputs, Level 2 [Member]
Assets:
Derivative assets		67	36
Liabilities:
Derivative liabilities		97	163
Interest rate swaps [Member] | Fair Value, Recurring [Member] | Fair Value, Inputs, Level 3 [Member]
Assets:
Derivative assets		0	0
Liabilities:
Derivative liabilities		0	0
Estimate of Fair Value Measurement [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Trading Securities		1,556	55
Assets:
Available-for-sale Securities		777	865
Derivative assets		368	314
Grantor trust assets (included in Other assets)		52	52
Liabilities:
Derivative liabilities		9	17
Estimate of Fair Value Measurement [Member] | Fair Value, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Trading Securities		1,556
Assets:
Grantor trust assets (included in Other assets)		52	52
Total recurring assets at fair value		2,753	1,286
Liabilities:
Total recurring liabilities at fair value		9	17
Estimate of Fair Value Measurement [Member] | Fair Value, Recurring [Member] | US Treasury Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Trading Securities		1,498
Estimate of Fair Value Measurement [Member] | Fair Value, Recurring [Member] | Government-Sponsored Enterprises Debt Obligations [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Trading Securities		58	55
Estimate of Fair Value Measurement [Member] | Fair Value, Recurring [Member] | Mortgage-backed Securities, Private-label residential [Member]
Assets:
Available-for-sale Securities		777	865
Estimate of Fair Value Measurement [Member] | Interest rate swaps [Member] | Fair Value, Recurring [Member]
Assets:
Derivative assets		368	314
Liabilities:
Derivative liabilities		$ 9	$ 17

[1] Amounts represent the application of the netting requirements that allow the Bank to settle positive and negative positions, and also cash collateral and related accrued interest held or placed with the same clearing agents and/or counterparty. Cash collateral posted and related accrued interest was $411 and $433 as of June 30, 2019 and December 31, 2018, respectively. Cash collateral received and related accrued interest was $22 and $9 as of June 30, 2019 and December 31, 2018, respectively.

[2] Amounts represent the application of the netting requirements that allow the Bank to settle positive and negative positions, and also cash collateral and related accrued interest held or placed with the same clearing agents and/or counterparty.