Organization and Summary of Significant Accounting Policies (Schedule of Aggregate Cost Basis and Depreciated Basis for Federal Income Tax Purposes of Investment in Real Estate) (Detail) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Accounting Policies [Abstract]
Aggregate cost basis for federal income tax purposes	$ 3,174.7	$ 3,303.2
Aggregate depreciated basis for federal income tax purposes	$ 2,284.0	$ 2,380.8