Summary of Significant Accounting Policies Leasing (Details) $ in Millions	12 Months Ended
	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)
New Accounting Pronouncement Not Yet Adopted (Leases) [Abstract]
Expense impact of costs attributable to successful leasing activities	$ 12.3	$ 13.9
Number of Office and Ground Leases	20
Operating Lease, Right-of-Use Asset	$ 40.0