Notes payable - Additional Information (Detail) (USD $)	Mar. 31, 2015	Dec. 31, 2014
Fixed Rate 700 Percent Notes Due July 2019 [Member]
Debt Disclosure [Line Items]
Senior Long Term Notes	$ 450,000,000	$ 450,000,000
Federal Reserve Bank Advances [Member]
Debt Disclosure [Line Items]
Line Of Credit Facility Current Borrowing Capacity	$ 2,100,000,000	$ 2,100,000,000

Carrying value as of the balance sheet date of Notes with the highest claim on the assets of the issuer in case of bankruptcy or liquidation (with maturities initially due after one year or beyond the operating cycle if longer), excluding current portion. Senior note holders are paid off in full before any payments are made to junior note holders.

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 02

-Paragraph 22

-Article 5

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 210

-SubTopic 10

-Section S99

-Paragraph 1

-Subparagraph (SX 210.5-02.22)

-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

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