Stock-based Compensation (Summary of Share Based Compensation Expense) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation expense	$ 31,722	$ 32,829	$ 33,979
Out Performance Units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation expense	17,763	20,630	21,136
Out Performance Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation expense	10,689	10,723	11,055
AO LTIP Units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation expense	2,113	0	0
Employee Stock Options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation expense	587	747	937
Restricted stock
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation expense	$ 570	$ 729	$ 851