Deposit Accounts - Scheduled Maturities for Certificates of Deposit (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Contractual Maturities, Certificates of Deposit, $100,000 or More
Three months or less	$ 223	$ 251
Over three months to six months	238	165
Over six months to twelve months	278	229
One to two years	101	139
Thereafter	35	33
Total	$ 875	$ 817

Contractual Maturities, Time Deposits, $250,000 or More, 12 Months Through 24 Months

No definition available.

Name:	fbc_ContractualMaturitiesTimeDeposits250000orMore12MonthsThrough24Months
Namespace Prefix:	fbc_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

Contractual Maturities, Time Deposits, $250,000 or More, Six Months Through 12 Months

No definition available.

Name:	fbc_ContractualMaturitiesTimeDeposits250000orMoreSixMonthsThrough12Months
Namespace Prefix:	fbc_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

Contractual Maturities, Time Deposits, $250,000 or More, Three Months Through Six Months

No definition available.

Name:	fbc_ContractualMaturitiesTimeDeposits250000orMoreThreeMonthsThroughSixMonths
Namespace Prefix:	fbc_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

Contractual Maturities, Time Deposits, $250,000 or More, Three Months or Less

No definition available.

Name:	fbc_ContractualMaturitiesTimeDeposits250000orMoreThreeMonthsorLess
Namespace Prefix:	fbc_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

Contractual Maturities, Time Deposits, $250,000 or More, after 24 Months

No definition available.

Name:	fbc_ContractualMaturitiesTimeDeposits250000orMoreafter24Months
Namespace Prefix:	fbc_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

Time Deposits, $250,000 or More

No definition available.

No definition available.

Name:	us-gaap_ContractualMaturitiesTimeDeposits100000OrMoreAbstract
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration