Stock-Based Compensation- Restricted Stock and Restricted Stock Units Activity (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended	12 Months Ended
Mar. 31, 2016	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Obligations, Fair Value Disclosure		$ 11	$ 12
Non-vested shares outstanding, Beginning balance	2,774,000	2,774,000	2,847,000	3,705,000
Granted		1,426,000	1,450,000	803,000
Forfeited		(294,000)	(464,000)	(485,000)
Vested		(1,473,000)	(1,059,000)	(1,176,000)
Non-vested shares outstanding, Ending balance		2,433,000	2,774,000	2,847,000
Performance Market Based Shares [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Non-vested shares outstanding, Beginning balance	1,074,000	1,074,000	945,000	706,000
Granted	436,000	436,000	414,000	338,000
Forfeited		(77,000)	(97,000)	(69,000)
Vested		(574,000)	(188,000)	(30,000)
Non-vested shares outstanding, Ending balance		859,000	1,074,000	945,000
Non-vested shares outstanding, Weighted Average Grant Date Fair Value per Share, Beginning balance	$ 23.76	$ 23.76	$ 28.61	$ 34.22
Granted, Weighted Average Grant Date Fair Value per Share		28.94	22.85	18.59
Forfeited, Weighted Average Grant Date Fair Value per Share		43.04	36.72	27.56
Vested, Weighted Average Grant Date Fair Value per Share		21.36	39.42	39.43
Non-vested shares outstanding, Weighted Average Grant Date Fair Value per Share, Ending balance		$ 26.28	$ 23.76	$ 28.61
Performance Market Based Shares [Member] \| Minimum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Rights, Percentage	0.00%
Performance Market Based Shares [Member] \| Maximum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Rights, Percentage	200.00%
Service - Based Shares [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Non-vested shares outstanding, Beginning balance	1,700,000	1,700,000	1,902,000	2,999,000
Granted		990,000	1,036,000	465,000
Forfeited		(217,000)	(367,000)	(416,000)
Vested		(899,000)	(871,000)	(1,146,000)
Non-vested shares outstanding, Ending balance		1,574,000	1,700,000	1,902,000
Non-vested shares outstanding, Weighted Average Grant Date Fair Value per Share, Beginning balance	$ 30.30	$ 30.30	$ 30.79	$ 33.45
Granted, Weighted Average Grant Date Fair Value per Share		37.95	31.20	30.40
Forfeited, Weighted Average Grant Date Fair Value per Share		29.15	21.69	28.20
Vested, Weighted Average Grant Date Fair Value per Share		29.34	32.10	36.65
Non-vested shares outstanding, Weighted Average Grant Date Fair Value per Share, Ending balance		$ 35.56	$ 30.30	$ 30.79
Cash-Settled Restricted Stock Units [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Obligations, Fair Value Disclosure		$ 11
Non-vested shares outstanding, Beginning balance	708,000	708,000	1,216,000	1,170,000
Granted		299,000	211,000	759,000
Forfeited		(101,000)	(257,000)	(126,000)
Vested		(446,000)	(462,000)	(587,000)
Non-vested shares outstanding, Ending balance		460,000	708,000	1,216,000

Fair value of obligations measured on a recurring basis.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=66048111&loc=d3e19207-110258

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
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-SubTopic 10
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The number of equity-based payment instruments, excluding stock (or unit) options, that were forfeited during the reporting period.

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The number of non-vested equity-based payment instruments, excluding stock (or unit) options, that validly exist and are outstanding as of the balance sheet date.

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