Stock-based Compensation (Out Performance Plans Narrative) (Details) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation expense	$ 32,829,000	$ 33,980,000	$ 39,846,000
Full Value Awards
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares authorized (in shares)	2,353,000
Full Value Awards | Maximum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares authorized (in shares)	6,000,000
Not Full Value Awards
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares authorized (in shares)	12,000,000
Out Performance Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation expense	$ 10,723,000	$ 11,055,000	15,531,000
Out Performance Plan
Award vesting in three years	33.33%
Award vesting in four years	33.33%
Award vesting in five years	33.33%
Compensation cost not yet recognized	$ 4,159,000
Compensation cost not yet recognized, period for recognition	1 year 8 months
Out Performance Plan 2017
Out Performance Plan
Out of performance plan notional amount	$ 35,000,000
Percentage of Notional Amount Granted	86.60%
Grant-date fair value	$ 10,800,000
OPP Units Earned, String	To be determined in 2020
Out Performance Plan 2016
Out Performance Plan
Out of performance plan notional amount	$ 40,000,000
Percentage of Notional Amount Granted	86.70%
Grant-date fair value	$ 11,800,000
OPP Units Earned, String	To be determined in 2019
Award Amortization Period	5 years
Out Performance Plan 2016 | Absolute Component
Out Performance Plan
Required shareholder return per year	7.00%
Required shareholder return three year	21.00%
Duration of performance measurement period	3 years
Out Performance Plan 2016 | Relative Component
Out Performance Plan
Duration of performance measurement period	3 years
Required shareholder return under relative component	6.00%
Percentage of shareholder return under which no awards will be earned	0.00%
Out Performance Plan 2015
Out Performance Plan
Out of performance plan notional amount	$ 40,000,000
Percentage of Notional Amount Granted	84.50%
Grant-date fair value	$ 9,120,000
OPP Units Earned, String	Not earned
Out Performance Plan 2014
Out Performance Plan
Awards earned (in percentage)	99.50%
Out Performance Plan 2013 to 2015
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Additional expense from acceleration of vesting			$ 7,834,000
Revised age limit for awards vesting criteria			65 years