Dividends declared (Details) (USD $) In Millions, except Per Share data, unless otherwise specified	3 Months Ended
Mar. 31, 2015	Feb. 06, 2015	Nov. 05, 2014	Aug. 05, 2014	Apr. 28, 2014	Feb. 18, 2014
Dividends declared/paid [Abstract]
Minimum percentage of taxable income to be distributed as REIT	90.00%
Distribution Per Share		$ 0.62us-gaap_DividendsPayableAmountPerShare	$ 0.62us-gaap_DividendsPayableAmountPerShare	$ 0.57us-gaap_DividendsPayableAmountPerShare	$ 0.57us-gaap_DividendsPayableAmountPerShare	$ 0.57us-gaap_DividendsPayableAmountPerShare
Aggregate Payment Amount (in millions)		$ 46.0	$ 46.0	$ 41.4	$ 41.5	$ 41.1

Carrying value as of the balance sheet date of dividends declared but unpaid on equity securities issued by the entity and outstanding.

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 210.9-03.15(5))

-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

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

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 03

-Paragraph 15

-Subparagraph a

-Article 7

Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 03

-Paragraph 15

-Subparagraph 5

-Article 9

Reference 4: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 944

-SubTopic 210

-Section S99

-Paragraph 1

-Subparagraph (SX 210.9-03.15(5))

-URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910

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