Fair Value Measurements - Assets and Liabilities Measured at Fair Value on Non-Recurring Basis (Details) - USD ($) $ in Millions	Jun. 30, 2021		Dec. 31, 2020		Dec. 31, 2019
Financial Assets:
Cash and cash equivalents, Carrying Value	$ 2,086		$ 1,948		$ 742
Cash and cash equivalents, Estimated Fair Value	2,086		1,948
FHLB stock, Carrying Value	686	[1]	714	[2]
FHLB stock, Estimated Fair Value	686	[1]	714	[2]
Loans, net, Carrying Value	43,373		42,807
Loans, net, Estimated Fair Value	42,913		42,376
Financial Liabilities:
Deposits, Carrying Value	34,177		32,437
Deposits, Estimated Fair Value	34,186		32,466
Borrowed funds, Carrying Value	15,459		16,084
Borrowed funds, Estimated Fair Value	16,243		16,794
Quoted Prices in Active Markets for Identical Assets (Level 1)
Financial Assets:
Cash and cash equivalents, Carrying Value	2,086		1,948
Financial Liabilities:
Deposits, Carrying Value	25,228	[3]	22,106	[4]
Significant Other Observable Inputs (Level 2)
Financial Assets:
FHLB stock, Carrying Value	686	[1]	714	[2]
Financial Liabilities:
Deposits, Carrying Value	8,958	[5]	10,360	[6]
Borrowed funds, Carrying Value	16,243		16,794
Significant Unobservable Inputs (Level 3)
Financial Assets:
Loans, net, Carrying Value	$ 42,913		$ 42,376

[1] Carrying value and estimated fair value are at cost.

[2] Carrying value and estimated fair value are at cost.

[3] Interest-bearing checking and money market accounts, savings accounts, and non-interest-bearing accounts.

[4] Interest-bearing checking and money market accounts, savings accounts, and non-interest-bearing accounts.

[5] Certificates of deposit.

[6] Certificates of deposit.