Contingent liabilities and commitments (Details) $ in Millions	Dec. 31, 2017 USD ($)
Loss Contingencies [Line Items]
Guarantee of indebtedness, relating to unused letters of credit	$ 5.4
Escrow Account Deposit
Loss Contingencies [Line Items]
Contingent liability, escrow deposit	1,200.0
Escrow Account Deposit - Section 1031 Exchange
Loss Contingencies [Line Items]
Contingent liability, escrow deposit	$ 1,100.0

The designation of funds furnished by a borrower to a lender to assure future payments of the borrower's real estate taxes and insurance obligations with respect to a mortgaged property. Escrow deposits may be made for a variety of other purposes such as earnest money and contingent payments. This element excludes replacement reserves which are an escrow separately provided for within the US GAAP taxonomy.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03.10)
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

Name:	us-gaap_EscrowDeposit
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant