Subsequent Events (Details) £ in Millions, $ in Millions	Apr. 30, 2018 USD ($) $ / £	Apr. 30, 2018 GBP (£) $ / £	Dec. 31, 2017 USD ($)	Feb. 28, 2013 USD ($) $ / £	Feb. 28, 2013 GBP (£) $ / £
Subsequent Event [Line Items]
Sale consideration of sold property | $			$ 1,000.0
One Westferry Circus
Subsequent Event [Line Items]
Foreign currency exchange rate, translation (in usd per share)				1.51	1.51
Business acquisition, contract purchase price				$ 124.6	£ 82.0
Subsequent Event | One Westferry Circus
Subsequent Event [Line Items]
Sale consideration of sold property	$ 153.5	£ 108.6
Foreign currency exchange rate, translation (in usd per share)	1.41	1.41