Long-Term Debt (Revolving Credit Facility) (Details) - USD ($) $ in Thousands	Mar. 31, 2017	Dec. 31, 2016
Line of Credit Facility [Line Items]
Principal amount	$ 3,602,889	$ 3,677,889
Line of Credit [Member] | Revolving Credit Facility [Member]
Line of Credit Facility [Line Items]
Principal amount	$ 250,000	$ 325,000
Line of Credit Facility, Interest Rate at Period End	2.34%
Letters of credit outstanding	$ 76,493
Remaining borrowing capacity	$ 673,507