Condensed Consolidating Financial Statements (Narrative) (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Debt Instrument [Line Items]
Long-term Debt, Gross	$ 3,150,000	$ 3,150,000
Parent Percentage of Ownership in Guarantor Subsidiaries	100.00%
4.75% Senior Notes [Member]
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage	4.75%	4.75%
5.00% Senior Notes [Member]
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage	5.00%	5.00%
4.75% Senior Notes Due 2025 [Member]
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage	4.75%	4.75%
Senior Notes [Member] | 4.75% Senior Notes [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross	$ 600,000	$ 600,000
Senior Notes [Member] | 5.00% Senior Notes [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross	1,000,000	1,000,000
Senior Notes [Member] | 4.75% Senior Notes Due 2025 [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross	$ 800,000	$ 800,000