Fair Value Measurement - Additional Information (Details) - USD ($)	Sep. 30, 2019	Sep. 30, 2018
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Abstract]
Fair value, assets, to or from level 3 transfers, amount	$ 0	$ 0
Fair value, liabilities, to or from level 3 transfers, amount	0	0
Fair value, equity, to or from level 3 transfers, amount	$ 0	$ 0

Fair value liabilities to or from level three transfers amount.

No definition available.

Name:	by_FairValueLiabilitiesToOrFromLevelThreeTransfersAmount
Namespace Prefix:	by_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant