Additional Shareholders' Equity Information (Details) - USD ($) $ / shares in Units, $ in Millions			3 Months Ended	12 Months Ended
	Mar. 25, 2015	Mar. 03, 2015	Mar. 31, 2015	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Components And Changes In Accumulated Other Comprehensive Income [Line Items]
Common Stock, par value per share (in dollars per share)				$ 0.01	$ 0.01
Proceeds from Issuance of Shares of Common Stock to Public, Net of Offering Costs				$ 1,112	$ 0	$ 0
Repayments of Long-term Lines of Credit				$ 70	$ 1,050	$ 900
Common stock shares issued [Rollforward]
Shares, beginning of period (in shares)			402,329,325	402,329,325		399,841,717
Exercise of common stock options (in shares)				343,145.000000	1,459,490
Restricted stock awards, net of forfeitures (in shares)				1,847,802	1,028,118
Public Equity Offering (in shares)				24,150,000	0
Shares exchanged in Rosetta Merger (in shares)				41,048,240	0
Shares, end of period (in shares)				469,718,512	402,329,325
Treasury stock [Rollforward]
Shares, beginning of period (in shares)			37,635,890	37,635,890		37,600,051
Shares received from employees in payment of withholding taxes due on vesting of shares of restricted stock (in shares)				490,744	254,888
Rabbi Trust Shares Distributed and/or Sold (in shares)				(201,009)	(219,049)
Shares, end of period (in shares)				37,925,625	37,635,890
Accumulated Other Comprehensive Loss
Beginning Balance			$ (90)	$ (90)	$ (117)	$ (113)
Realized Amounts Reclassified Into Earnings				63	12	13
Unrealized Change in Fair Value				(6)	15	(17)
End Balance				$ (33)	$ (90)	$ (117)
Effective income tax rate applied to AOCI (in hundredths)				35.00%	35.00%	35.00%
Deferred loss				$ (22)
Interest Rate Cash Flow Hedges
Accumulated Other Comprehensive Loss
Beginning Balance			(23)	(23)	$ (24)	$ (25)
Realized Amounts Reclassified Into Earnings				1	1	1
Unrealized Change in Fair Value				0	0	0
End Balance				(22)	(23)	(24)
Pension- Related and Other
Accumulated Other Comprehensive Loss
Beginning Balance			$ (67)	(67)	(93)	(88)
Realized Amounts Reclassified Into Earnings				62	11	12
Unrealized Change in Fair Value				(6)	15	(17)
End Balance				(11)	$ (67)	$ (93)
Underwritten Public Offering
Components And Changes In Accumulated Other Comprehensive Income [Line Items]
Common Stock, par value per share (in dollars per share)		$ 0.01
Sales price (in dollars per share)		$ 47.50
Proceeds from Issuance of Shares of Common Stock to Public, Net of Offering Costs				1,100
Repayments of Long-term Lines of Credit				$ 150
Common stock shares issued [Rollforward]
Public Equity Offering (in shares)		21,000,000	24,150,000
Over-Allotment Option
Components And Changes In Accumulated Other Comprehensive Income [Line Items]
Common Stock, par value per share (in dollars per share)	$ 0.01
Common stock shares issued [Rollforward]
Public Equity Offering (in shares)	3.15