Summary of Significant Accounting Policies (Fair Value of Financial Instruments) (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Carrying Amount
Financial assets:
Cash and cash equivalents	$ 144,472	$ 29,054
Deferred compensation plan assets	24,403	22,617
Loan receivable	13,497	13,000
Financial liabilities:
Senior unsecured credit facility	525,000	1,055,000
Senior unsecured notes	4,031,457	3,425,000
Fair Value
Financial assets:
Cash and cash equivalents	144,472	29,054
Deferred compensation plan assets	24,403	22,617
Loan receivable	13,497	13,000
Financial liabilities:
Senior unsecured credit facility	515,183	1,045,600
Senior unsecured notes	$ 4,050,052	$ 3,574,688