Summary of Significant Accounting Policies (Fair Value of Financial Instruments) (Details) - USD ($) $ in Thousands	Jun. 30, 2017	Dec. 31, 2016
Carrying Amount
Financial assets:
Cash and cash equivalents	$ 29,510	$ 36,556
Deferred compensation plan assets	20,150	17,593
Loan receivable	13,000	26,200
Financial liabilities:
Senior unsecured credit facility	1,140,000	1,290,000
Senior unsecured notes	3,425,000	3,425,000
Fair Value
Financial assets:
Cash and cash equivalents	29,510	36,556
Deferred compensation plan assets	20,150	17,593
Loan receivable	13,000	26,200
Financial liabilities:
Senior unsecured credit facility	1,133,435	1,272,852
Senior unsecured notes	$ 3,646,375	$ 3,573,500