Summary of Significant Accounting Policies Deferred Financing and Leasing Costs (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Accounting Policies [Abstract]
Capitalized compensation, leasing costs	$ 3.6	$ 3.8	$ 4.6