Loans and asset quality - Lost Interest (Detail) - USD ($) $ in Millions	3 Months Ended
Mar. 31, 2016	Dec. 31, 2015	Mar. 31, 2015
Receivables [Abstract]
Amount by which interest income recognized on nonperforming loans exceeded reversals	$ 0	$ 0	$ 0
Amount by which interest income would have increased if nonperforming loans at year-end had been performing for the entire period	$ 1	$ 2	$ 2

Reflects the amount of additional interest income that would have been recorded if impaired or nonperforming loans were instead current, in compliance with their original terms, and outstanding throughout the reporting period or since origination (if held for part of the period).

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 40
-Section 50
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=25495931&loc=d3e18794-111554

Name:	us-gaap_LoansAndLeasesReceivableImpairedInterestLostOnNonaccrualLoans
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration