Subsequent Events (Details) - Reorganization Transactions on April 1, 2016 $ in Millions	Apr. 01, 2016 USD ($)
Subsequent Events
Certain tax related liabilities incurred	$ 95.4
BancWest Corporation \| Bank of the West \| BancWest Holding Inc.
Subsequent Events
Ownership prior to transactions (as a percent)	100.00%
Subsequent Events
Subsequent Events
Certain tax related liabilities incurred	$ 95.4
Subsequent Events \| BancWest Corporation \| Bank of the West \| BancWest Holding Inc.
Subsequent Events
Ownership prior to transactions (as a percent)	100.00%

Carrying value as of the balance sheet date of obligations incurred and payable for statutory income, sales, use, payroll, excise, real, property and other taxes. Used to reflect the current portion of the liabilities (due within one year or within the normal operating cycle if longer).

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.19,20)
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Name:	us-gaap_TaxesPayableCurrent
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant