Shareholders' Equity - Dividends Declared (Details) - Dividend Paid - USD ($) $ / shares in Units, $ in Millions	Oct. 12, 2017	Jul. 10, 2017	Apr. 25, 2017	Feb. 06, 2017	Oct. 18, 2016	Jul. 20, 2016	Apr. 20, 2016	Feb. 03, 2016	Nov. 03, 2015	Jul. 31, 2015	Apr. 29, 2015	Feb. 06, 2015
Dividends Payable [Line Items]
Distribution Per Share (in dollars per share)	$ 0.47	$ 0.47	$ 0.47	$ 0.47	$ 0.43	$ 0.43	$ 0.43	$ 0.43	$ 0.43	$ 0.41	$ 0.41	$ 0.41
Aggregate Payment Amount	$ 58.3	$ 58.3	$ 58.4	$ 52.5	$ 48.8	$ 48.7	$ 48.7	$ 48.5	$ 48.5	$ 46.3	$ 46.3	$ 46.0
Qualified
Dividends Payable [Line Items]
Distribution Per Share (in dollars per share)	$ 0.0176751	$ 0.0176751	$ 0.0176751	$ 0.0175622	$ 0.0493613	$ 0.0493613	$ 0.0493613	$ 0.0493613	$ 0.0370254	$ 0.0353166	$ 0.0353166	$ 0.0353166
Non-Qualified
Dividends Payable [Line Items]
Distribution Per Share (in dollars per share)	0.2484259	0.2484259	0.2484259	0.2468402	0.2886402	0.2886402	0.2886402	0.2886402	0.2893354	0.2759814	$ 0.2759814	$ 0.2759814
Nondividend Distributions
Dividends Payable [Line Items]
Distribution Per Share (in dollars per share)	$ 0.2038990	$ 0.2038990	$ 0.2038990	$ 0.2025975	$ 0.0953319	$ 0.0953319	$ 0.0953319	$ 0.0953319	$ 0.1069725	$ 0.1020353