Financial Instruments and Derivatives Fair Value, by Balance Sheet Grouping (Details) - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Preferred Stock, Redemption Date	Oct. 01, 2026
Preferred Stock, Redemption Price Per Share	$ 50.00
Fixed Rate Debt
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Notes Payable Excluding Interest Rate Swaps and Cap Agreements	$ 3,100.0	$ 3,200.0
Notes Payable, Fair Value Disclosure	3,100.0	3,300.0
Floating Rate Debt
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Notes Payable Excluding Interest Rate Swaps and Cap Agreements	1,100.0	1,300.0
Notes Payable, Fair Value Disclosure	1,100.0	1,300.0
Fair Value, Measurements, Recurring [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Interest Rate Derivative Assets, at Fair Value	3.7	3.6
Interest Rate Derivative Liabilities, at Fair Value	5.3	1.3
Embedded Derivative, Fair Value of Embedded Derivative Asset	$ 18.6	$ 21.2