Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Millions		Jun. 30, 2018	Dec. 31, 2017
Assets
Related party accounts receivable		$ 3.4	$ 5.2
Fair value of unconsolidated investments		478.3	380.7
Assets	[1]	7,505.4	7,724.8
Real estate and acquired in place lease values, net of accumulated depreciation and amortization		6,097.7	6,443.7
Liabilities
Liabilities	[1]	$ 6,055.7	$ 6,147.3
Stockholders' Equity Attributable to Parent [Abstract]
Common stock, shares issued (in shares)		144,405,875	151,561,284
Common stock, shares outstanding (in shares)		144,405,875	151,561,284
Variable Interest Entity, Primary Beneficiary [Member]
Assets
Assets		$ 642.7	$ 904.4
Cash held by consolidated investments		27.5	39.1
Real estate and acquired in place lease values, net of accumulated depreciation and amortization		565.8	789.1
Liabilities
Liabilities		335.5	510.0
Variable Interest Entity, Primary Beneficiary [Member] | Investment Debt [Member]
Liabilities
Long-term debt		$ 301.5	$ 475.3

[1] The assets and liabilities as of June 30, 2018 include $642.7 million (including cash held by consolidated investments of $27.5 million and real estate and acquired in place lease values, net of accumulated depreciation and amortization of $565.8 million) and $335.5 million (including investment debt of $301.5 million), respectively, from consolidated variable interest entities ("VIEs"). The assets and liabilities as of December 31, 2017 include $904.4 million (including cash held by consolidated investments of $39.1 million and real estate and acquired in place lease values, net of accumulated depreciation and amortization of $789.1 million) and $510.0 million (including investment debt of $475.3 million), respectively, from VIEs. These assets can only be used to settle obligations of the consolidated VIEs, and the liabilities do not have recourse to the Company.