Capital Structure (Common Stock - Details 2) - USD ($) $ / shares in Units, $ in Millions		3 Months Ended													12 Months Ended				13 Months Ended
		Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2015
Employee Stock Purchase Plan and 2013 Capital Allocation Plan
Common Stock, Dividends, Per Share, Cash Paid			$ 0.00030	$ 0.00030	$ 0.00030	$ 0.00145	$ 0.00145	$ 0.00145	$ 0.00145	$ 0.00145	$ 0.00140	$ 0.00140	$ 0.00140	$ 0.00120				$ 0.14
Common Stock, Dividends, Proposed Annual Percentage Increase			79.00%				4.00%									79.00%	4.00%		4.00%
Dividends Per Common Share																$ 0.24	$ 0.58	$ 0.54
Eligible compensation (as a percent)																10.00%
Exercise price as a percentage of fair value (as a percent)																85.00%
Treasury stock reserved for issuance under the ESPP (in shares)			667,819													667,819
Commissions per Share																			$ 0.015
Common Stock
Employee Stock Purchase Plan and 2013 Capital Allocation Plan
Common stock issued to employee from treasury stock (in shares)																609,094	283,139	128,336
Stock Repurchased During Period, Shares																	(24,189,495)	(1,624,360)
Scenario, Plan [Member]
Employee Stock Purchase Plan and 2013 Capital Allocation Plan
Common Stock, Dividends, Proposed Annual Amount, Per Share																$ 0.12	$ 0.0058	$ 0.56
Subsequent Event [Member]
Employee Stock Purchase Plan and 2013 Capital Allocation Plan
Dividends Payable, Date Declared		Jan. 18, 2017
Dividends Per Common Share		$ 0.0003
Dividends Payable, Date to be Paid		Feb. 15, 2017
Dividends Payable, Date of Record		Feb. 01, 2017
Subsequent Event [Member] | Common Stock
Employee Stock Purchase Plan and 2013 Capital Allocation Plan
Common stock issued to employee from treasury stock (in shares)		282,530
Subsequent Event [Member] | Scenario, Plan [Member]
Employee Stock Purchase Plan and 2013 Capital Allocation Plan
Common Stock, Dividends, Proposed Annual Amount, Per Share															$ 0.0012
Long-term incentive plans
Employee Stock Purchase Plan and 2013 Capital Allocation Plan
Common Stock, Capital Shares Reserved for Future Issuance			17,336,092													17,336,092
Capital Allocation Plan [Member]
Employee Stock Purchase Plan and 2013 Capital Allocation Plan
Stock Repurchase Program, Authorized Amount								$ 481
Stock Repurchased During Period, Shares							5,558,920	11,104,184	4,379,907	3,146,484	1,624,360								25,813,855
Treasury Stock Acquired, Average Cost Per Share	[1]						$ 15.03	$ 15.06	$ 24.53	$ 25.15	$ 26.95
Stock Repurchased During Period, Value							$ 84	$ 167	$ 107	$ 79	$ 44								$ 481

[1] The average price paid per share and amounts paid for shares purchased exclude the commissions of $0.015 per share paid in connection with the share repurchase.