Stock-based Compensation (Narratives) (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Share Based Compensation Expense	$ 39,846	$ 36,641	$ 34,914
Corporate general and administrative expenses in 2016 due to accelerated compensation expenses	2,940
Corporate general and administrative expenses after 2016 due to accelerated compensation expenses	$ 3,277
Full Value Awards [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Share Based Compensation Arrangement By Share Based Payment Award Number Of Shares Authorized (in Shares)	6,000,000
Share Based Compensation Shares Available For Grant (in Shares)	3,570,000
Not Full Value Awards [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Share Based Compensation Arrangement By Share Based Payment Award Number Of Shares Authorized (in Shares)	12,000,000
Out Performance Plan [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Share Based Compensation Expense	$ 15,531	6,185	3,226
Additional Expense From Acceleration Of Vesting	$ 6,217
Revised Age Limit For Awards Vesting Criteria	65 years
Out Performance Plan [Abstract]
Award vesting in three years	33.00%
Award versting in four years	33.00%
Award vesting in five years	34.00%
Compensation Cost Not Yet Recognized	$ 5,087
Compensation Cost Recognition Period	1 year 8 months 12 days
Out Performance Plan 2015 [Member]
Out Performance Plan [Abstract]
Required award holding period after vesting	1 year
Grant-Date Fair Value	$ 9,120
Award Amortization Period	5 years
Out Performance Plan 2015 [Member] | Absolute [Member]
Out Performance Plan [Abstract]
Duration Of Performance Measurement Period	3 years
Required Shareholder Return Per Year	7.00%
Required Shareholder Return Three Year	21.00%
Out Performance Plan 2015 [Member] | Relative [Member]
Out Performance Plan [Abstract]
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 2014 [Member]
Out Performance Plan [Abstract]
Required award holding period after vesting	1 year
Grant-Date Fair Value	$ 8,202
Award Amortization Period	5 years
Out Performance Plan 2014 [Member] | Absolute [Member]
Out Performance Plan [Abstract]
Duration Of Performance Measurement Period	3 years
Required Shareholder Return Per Year	7.00%
Required Shareholder Return Three Year	21.00%
Out Performance Plan 2014 [Member] | Relative [Member]
Out Performance Plan [Abstract]
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 2013 [Member]
Out Performance Plan [Abstract]
Required award holding period after vesting	1 year
Grant-Date Fair Value	$ 6,814
Award Amortization Period	5 years
Out Performance Plan 2012 [Member]
Out Performance Plan [Abstract]
Grant-Date Fair Value	$ 12,250
Award Amortization Period	5 years
Employee Stock Option [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Share Based Compensation Expense	$ 1,298	$ 4,550	$ 8,234
Out Performance Plan [Abstract]
Compensation Cost Not Yet Recognized	$ 1,325
Compensation Cost Recognition Period	1 year 8 months 12 days
Stock options expiration term	10 years
Award Vesting Period	4 years
Share Based Compensation Employee Stock Purchase Plan Activity [Abstract]
Grant Date Fair Value Of Options (in dollars per shares)	$ 28.85	$ 20.31	$ 17.18
Cash Received From Exercise Of Stock Options	$ 15,343	$ 17,441	$ 5,915
Total Intrinsic Value Of Options Excercised	3,873	18,223	3,386
Restricted Stock Units RSU [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Share Based Compensation Expense	837	1,303	1,344
Out Performance Plan [Abstract]
Compensation Cost Not Yet Recognized	$ 1,315
Compensation Cost Recognition Period	1 year 8 months 12 days
Dividends Paid On Nonvested Units	$ 58	88	110
Grant-Date Fair Value	$ 906	1,048	857
Award Vesting Period	4 years
Fair Value Of Awards Vested	$ 882	1,174	1,194
Restricted Operating Partnership Units Plan [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Share Based Compensation Expense	22,180	24,603	22,110
Out Performance Plan [Abstract]
Compensation Cost Not Yet Recognized	$ 18,625
Compensation Cost Recognition Period	1 year 7 months 6 days
Dividends Paid On Nonvested Units	$ 2,414	2,866	2,598
Grant-Date Fair Value	$ 20,293	19,669	31,947
Award Vesting Period	4 years
Fair Value Of Awards Vested	$ 20,072	$ 22,758	$ 16,404