Summary of Significant Accounting Policies - Narrative (Details) - USD ($) $ in Millions	6 Months Ended	12 Months Ended
	Jun. 30, 2020	Dec. 31, 2019	Mar. 31, 2020	Jan. 01, 2020	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Accounting Standards Update [Extensible List]	us-gaap:AccountingStandardsUpdate201613Member	us-gaap:AccountingStandardsUpdate201613Member
Increase to Distributions in excess of accumulated earnings	$ (1,104.6)	$ (1,128.7)	$ (1,110.8)		$ (1,192.6)	$ (1,205.5)	$ (1,208.3)
(Distribution in Excess of Accumulated Earnings) Earnings Surplus
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Increase to Distributions in excess of accumulated earnings	$ 643.4	626.2	$ 638.7		$ 554.0	$ 541.3	$ 538.9
Impact of ASC 326 Adoption
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Increase to Distributions in excess of accumulated earnings		4.1
Impact of ASC 326 Adoption | (Distribution in Excess of Accumulated Earnings) Earnings Surplus
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Increase to Distributions in excess of accumulated earnings		$ 4.0
ASU 2016-13 | Impact of ASC 326 Adoption | (Distribution in Excess of Accumulated Earnings) Earnings Surplus
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Increase to Distributions in excess of accumulated earnings				$ 4.0