Shareholders' Equity - Dividends declared (Details) - USD ($) $ / shares in Units, $ in Millions	6 Months Ended
	Jun. 30, 2015	Apr. 29, 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		$ 0.62	$ 0.62	$ 0.62	$ 0.57	$ 0.57	$ 0.57
Aggregate Payment Amount (in millions)		$ 46.3	$ 46.0	$ 46.0	$ 41.4	$ 41.5	$ 41.1