Common Stock and Related Matters - Dividends Declared And Or Paid (Details) - USD ($) $ / shares in Units, $ in Millions		3 Months Ended		9 Months Ended
		Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Dividends Payable [Line Items]
Accrued preferred dividends payable date				Nov. 14, 2018
Dividends Declared per Share of Common Stock		$ 0.91	$ 0.91	$ 2.73	$ 2.73
Dividend Declared, Q3 2018 [Member]
Dividends Payable [Line Items]
Accrued preferred dividends payable date				Nov. 15, 2018
Total Common Dividends Declared				$ 212.5
Amount of Common Dividends Paid or To Be Paid				208.6
Accrued Dividends	[1]	$ 3.9		$ 3.9
Dividends Declared per Share of Common Stock				$ 0.91000
Dividend Declared, Q2 2018 [Member]
Dividends Payable [Line Items]
Accrued preferred dividends payable date				Aug. 15, 2018
Total Common Dividends Declared				$ 208.9
Amount of Common Dividends Paid or To Be Paid				205.2
Accrued Dividends	[1]	3.7		$ 3.7
Dividends Declared per Share of Common Stock				$ 0.91000
Dividend Declared, Q1 2018 [Member]
Dividends Payable [Line Items]
Accrued preferred dividends payable date				May 16, 2018
Total Common Dividends Declared				$ 203.1
Amount of Common Dividends Paid or To Be Paid				199.7
Accrued Dividends	[1]	3.4		$ 3.4
Dividends Declared per Share of Common Stock				$ 0.91000
Dividend Declared, Q4 2017 [Member]
Dividends Payable [Line Items]
Accrued preferred dividends payable date				Feb. 15, 2018
Total Common Dividends Declared				$ 202.4
Amount of Common Dividends Paid or To Be Paid				199.1
Accrued Dividends	[1]	$ 3.3		$ 3.3
Dividends Declared per Share of Common Stock				$ 0.91000

[1] Represents accrued dividends on restricted stock and restricted stock units that are payable upon vesting.