Long-term Debt (Summary of Long-term Debt) (Details) - Senior Notes [Member] - USD ($)	Dec. 31, 2016	Dec. 31, 2015
4.375% Senior Notes [Member]
Debt Instrument [Line Items]
Interest rate	4.375%	4.375%
Face amount	$ 1,000,000,000
Long-term debt, fair value	$ 975,000,000	$ 0
5.50% Senior Notes [Member]
Debt Instrument [Line Items]
Interest rate	5.50%	5.50%
Face amount	$ 700,000,000.0
Long-term debt, fair value	$ 758,000,000	$ 718,000,000
5.875% Senior Notes [Member]
Debt Instrument [Line Items]
Interest rate	5.875%	5.875%
Face amount	$ 800,000,000
Long-term debt, fair value	$ 868,000,000	$ 820,000,000
5.750% Senior Notes [Member]
Debt Instrument [Line Items]
Interest rate	5.75%	5.75%
Face amount	$ 400,000,000
Long-term debt, fair value	$ 431,000,000	$ 411,000,000
5.375% Senior Notes [Member]
Debt Instrument [Line Items]
Interest rate	5.375%	5.375%
Face amount	$ 500,000,000
Long-term debt, fair value	$ 539,000,000	$ 525,000,000