Bank segment - Contingencies (Details) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017
American Savings Bank (ASB)
Loss Contingencies [Line Items]
Unfunded commitments to fund the company's LIHTC	$ 14.4	$ 15.8

Amount of commitment or contingent commitments to qualified affordable housing project investments.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 323
-SubTopic 740
-Section 50
-Paragraph 2
-Subparagraph (e)
-URI http://asc.fasb.org/extlink&oid=64813897&loc=SL49122993-202985

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