Dividends declared (Details) (USD $) In Millions, except Per Share data, unless otherwise specified	6 Months Ended
	Jun. 30, 2014	Apr. 28, 2014	Feb. 18, 2014	Nov. 01, 2013	Jul. 30, 2013	May 07, 2013	Jan. 17, 2013
Dividends declared/paid [Abstract]
Minimum Percentage Of Taxable Income To Be Distributed As REIT	90.00%
Distribution Per Share		$ 0.57	$ 0.57	$ 0.55	$ 0.5	$ 0.5	$ 0.5
Aggregate Payment Amount (in millions)		$ 41.5	$ 41.1	$ 39.6	$ 36.1	$ 35.8	$ 35.7