Stockholders' Equity (Narrative) (Details) - USD ($)	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2016	Dec. 31, 2015	Jun. 30, 2016	Jun. 30, 2015	Dec. 31, 2015	Dec. 31, 2012
Class of Stock [Line Items]
Total capital stock, shares authorized	400,000,000	400,000,000	400,000,000		400,000,000
Common stock, shares authorized	300,000,000	300,000,000	300,000,000		300,000,000
Preferred stock, shares authorized	100,000,000	100,000,000	100,000,000		100,000,000
Common stock, par value per share	$ 0.01	$ 0.01	$ 0.01		$ 0.01
Preferred stock, par value per share	$ 0.01	$ 0.01	$ 0.01		$ 0.01
Preferred stock, shares outstanding	0		0
Common Stock, Dividends, Per Share, Cash Paid						$ 0.25
Maximum amount of dividends or stock repurchases permissible under the Credit Facility			$ 200,000,000
Annual amount of dividends or stock repurchases permissible under the Credit Facility			25,000,000
Noncash Dividend	$ 695,000,000
Amount available for dividend payments, stock repurchases or Senior Notes repurchases at period end	$ 318,000,000		$ 318,000,000
Open Market Repurchase Program for Common Stock, Adopted November 6, 2015 [Member]
Class of Stock [Line Items]
Maximum number of shares authorized for repurchase	10,000,000		10,000,000
Maximum value of shares authorized under repurchase program	$ 300,000,000		$ 300,000,000
Number of shares repurchased and retired			0		532,188
Weighted-average price of repurchased and retired shares, per share					$ 27.31
Open Market Repurchase Program for Common Stock, Adopted December 10, 2014 [Member]
Class of Stock [Line Items]
Maximum number of shares authorized for repurchase	5,000,000		5,000,000
Maximum value of shares authorized under repurchase program	$ 150,000,000		$ 150,000,000
Number of shares repurchased and retired		5,000,000		0