Significant Accounting Policies - Adoption of New Guidance Impact on Previously Reported Quarterly Results (Details) - USD ($) $ / shares in Units, $ in Millions		3 Months Ended					6 Months Ended	9 Months Ended
		Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016		Sep. 30, 2015	Jun. 30, 2016	Sep. 30, 2016	Sep. 30, 2015
Consolidated Statement of Operations:
Operating income		$ 66				$ 88		$ 156	$ 257
Provision for income taxes		(8)				(24)		(27)	(70)
Net income		$ 55				$ 74		$ 119	$ 195
Basic EPS		$ 0.38				$ 0.51		$ 0.82	$ 1.35
Diluted EPS		$ 0.37				$ 0.51		$ 0.81	$ 1.34
Consolidated Statement of Cash Flows:
Net cash provided by operating activities								$ 277	$ 339
Net cash used in financing activities								$ (133)	(93)
New Accounting Pronouncement, Early Adoption | ASU 2016-09
Consolidated Statement of Cash Flows:
Net cash provided by operating activities									32
Net cash used in financing activities									$ (32)
New Accounting Pronouncement, Early Adoption | As Reported | ASU 2016-09
Consolidated Statement of Operations:
Operating income			$ 48	$ 42
Provision for income taxes			(11)	(11)
Net income			$ 34	$ 27
Basic EPS			$ 0.23	$ 0.19
Diluted EPS			$ 0.23	$ 0.18
Consolidated Statement of Cash Flows:
Net cash provided by operating activities				$ 120			$ 357
Net cash used in financing activities				(94)			(123)
New Accounting Pronouncement, Early Adoption | As Adjusted | ASU 2016-09
Consolidated Statement of Operations:
Operating income	[1]		$ 47	42	[2]
Provision for income taxes	[1]		(10)	(9)	[2]
Net income	[1]		$ 34	$ 29	[2]
Basic EPS	[1]		$ 0.23	$ 0.20	[2]
Diluted EPS	[1]		$ 0.23	$ 0.20	[2]
Consolidated Statement of Cash Flows:
Net cash provided by operating activities	[2]			$ 124	[1]		363
Net cash used in financing activities	[2]			$ (98)	[1]		$ (129)

[1] The election to account for forfeitures as they occur did not have a material impact for the three months ended March 31, 2016 and resulted in an increase to stock-based compensation expense of approximately $1 million for the three months ended June 30, 2016.

[2] Includes the reclassification of cash flows related to excess tax benefits from financing activities to operating activities of $4 million and $2 million for the three months ending March 31, 2016 and June 30, 2016, respectively.