Shareholders' Equity - Dividends Declared (Details) $ / shares in Units, $ in Millions	6 Months Ended
	Jun. 30, 2017	Apr. 25, 2017 USD ($)	Apr. 25, 2017 £ / shares	Feb. 06, 2017 USD ($)	Feb. 06, 2017 £ / shares	Oct. 18, 2016 USD ($)	Oct. 18, 2016 £ / shares	Jul. 20, 2016 USD ($) $ / shares	Apr. 20, 2016 USD ($) $ / shares	Feb. 03, 2016 USD ($) $ / shares
Equity [Abstract]
Minimum percentage of taxable income to be distributed as REIT	90.00%
Distribution Per Share (USD per share) | (per share)			£ 0.47		£ 0.47		£ 0.43	$ 0.43	$ 0.43	$ 0.43
Aggregate Payment Amount		$ 58.4		$ 52.5		$ 48.8		$ 48.7	$ 48.7	$ 48.5