Earnings Per Common Share Earnings per Common Share (Narrative) (Details)	Dec. 31, 2020 shares
Earnings Per Share [Abstract]
Dividend Equivalent Rights	3,023,314

Number of freestanding rights to share in dividends on a one-for-one basis

No definition available.

Name:	bku_DividendEquivalentRights
Namespace Prefix:	bku_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	instant