| **Stock-based Compensation - (Notes)**                                        | 12 Months Ended                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                        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| **Disclosure of Compensation Related Costs, Share-based Payments [Abstract]** |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                       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| Stock-based compensation                                                      | STOCK-BASED COMPENSATIONStock-based compensation cost is measured at the date of grant based on the calculated fair value of the award and is generally recognized on a straight-line basis over the vesting period of the equity grant.  The compensation cost is determined based on awards ultimately expected to vest; therefore, we have reduced the cost for estimated forfeitures based on historical forfeiture rates.  Forfeitures are estimated at the time of grant and revised, if necessary, in subsequent periods to reflect actual forfeitures.  There were no stock-based compensation costs capitalized as the amounts were not material.Stock-based compensation costs are as follows for the years ended December 31: 2015 2014 2013Stock-based compensation cost$120 $122 $115Tax benefit(28) (26) (24)Stock-based compensation cost, net of tax$92 $96 $91For our stock options and restricted stock awards and units, we currently have 60.7 million shares authorized for issuance and as of December 31, 2015, approximately 21.3 million shares were available for future grants.  Our policy is to issue new shares for exercises of stock options, when restricted stock awards are granted, at vesting of restricted stock units and for issuances under the employee stock purchase plan.Stock OptionsOur stock option plans provide for the issuance of stock options to directors, officers and other key employees at an exercise price equal to the fair market value of the stock at the date of grant.  Although subject to the terms of the stock option agreement, substantially all of the stock options become exercisable in three equal annual installments, beginning a year from the date of grant, and generally expire ten years from the date of grant.  The stock option plans provide for the acceleration of vesting upon the employee’s retirement; therefore, the service period is reduced for employees that are or will become retirement eligible during the vesting period, and accordingly, the recognition of compensation expense for these employees is accelerated.  No stock options were granted in 2015.The fair value of each stock option granted is estimated using the Black-Scholes option pricing model.  The following table presents the weighted average assumptions used in the option pricing model for options granted.  The expected life of the options represents the period of time the options are expected to be outstanding.  The expected life is based on our historical exercise trends and post-vest termination data incorporated into a forward-looking stock price model.  The expected volatility is based on our implied volatility, which is the volatility forecast that is implied by the prices of actively traded options to purchase our stock observed in the market.  The risk-free interest rate is based on the observed U.S. Treasury yield curve in effect at the time the options were granted.  The dividend yield is based on our history of dividend payouts.  2014 2013Expected life (years) 4.6 5.2Risk-free interest rate 1.5% 1.3%Volatility 31.9% 36.0%Dividend yield 1.0% 1.3%Weighted average fair value per share at grant date $16.81 $13.79The following table presents the changes in stock options outstanding and related information (in thousands, except per option prices): Number ofOptions Weighted AverageExercise PricePer OptionOutstanding at December 31, 2014 9,737   $53.80 Granted —   — Exercised (873)   44.25 Forfeited (124)   53.90 Expired (138)   67.85 Outstanding at December 31, 2015 8,602   $54.56 Exercisable at December 31, 2015 7,363   $54.46 The weighted average remaining contractual term for options outstanding and options exercisable at December 31, 2015 were 4.7 years and 4.2 years, respectively.The total intrinsic value of stock options (defined as the amount by which the market price of our common stock on the date of exercise exceeds the exercise price of the option) exercised in 2015, 2014 and 2013 was $15 million, $70 million and $11 million, respectively.  The income tax benefit realized from stock options exercised was $3.8 million, $19.6 million and $2.0 million in 2015, 2014 and 2013, respectively.The total fair value of options vested in 2015, 2014 and 2013 was $24 million, $29 million and $31 million, respectively.  As of December 31, 2015, there was $5 million of total unrecognized compensation cost related to unvested stock options, which is expected to be recognized over a weighted average period of one year.The total intrinsic value of stock options outstanding at December 31, 2015 was $15.1 million, of which $14.8 million relates to options vested and exercisable.  The intrinsic value for stock options outstanding is calculated as the amount by which the quoted price of $46.15 of our common stock as of the end of 2015 exceeds the exercise price of the options.Restricted Stock Awards and UnitsIn addition to stock options, our officers, directors and key employees may be granted restricted stock awards (“RSA”), which is an award of common stock with no exercise price, or restricted stock units (“RSU”), where each unit represents the right to receive, at the end of a stipulated period, one unrestricted share of stock with no exercise price.  RSAs and RSUs are subject to cliff or graded vesting, generally ranging over a three year period, or over a one year period for non-employee directors.  We determine the fair value of restricted stock awards and restricted stock units based on the market price of our common stock on the date of grant.  The following table presents the combined changes of RSAs and RSUs and related information (in thousands, except per award/unit prices): Number ofAwards and Units Weighted AverageGrant Date FairValue Per Award/UnitUnvested balance at December 31, 20142,732 $57.88Granted2,314 57.37Vested(1,299) 55.09Forfeited(391) 54.62Unvested balance at December 31, 20153,356 $58.99The weighted average grant date fair value per share for RSAs and RSUs granted in 2015, 2014 and 2013 was $57.37, $69.67 and $45.58, respectively.  The total fair value of RSAs and RSUs vested in 2015, 2014 and 2013 was $72 million, $60 million and $58 million, respectively.  As of December 31, 2015, there was $117 million of total unrecognized compensation cost related to unvested RSAs and RSUs, which is expected to be recognized over a weighted average period of two years.Employee Stock Purchase PlanThe Employee Stock Purchase Plan (“ESPP”) provides for eligible employees to purchase shares on an after-tax basis in an amount between 1% and 10% of their annual pay:  (i) on June 30 of each year at a 15% discount of the fair market value of our common stock on January 1 or June 30, whichever is lower, and (ii) on December 31 of each year at a 15% discount of the fair market value of our common stock on July 1 or December 31, whichever is lower.  An employee may not contribute more than $5,000 in either of the six-month measurement periods described above or $10,000 annually.We currently have 30.5 million shares authorized for issuance, and at December 31, 2015, there were 4.2 million shares reserved for future issuance.  Compensation cost for the years ended December 31, was calculated using the Black-Scholes option pricing model with the following assumptions: 2015 2014 2013Expected life (years)0.5 0.5 0.5Risk-free interest rate0.1% 0.03% 0.1%Volatility30.9% 24.7% 30.3%Dividend yield1.2% 1.0% 1.4%Fair value per share of the 15% cash discount$8.79 $9.72 $6.45Fair value per share of the look-back provision4.97 4.39 3.58Total weighted average fair value per share at grant date$13.76 $14.11 $10.03We calculated estimated volatility using historical daily prices based on the expected life of the stock purchase plan.  The risk-free interest rate is based on the observed U.S. Treasury yield curve in effect at the time the ESPP shares were granted.  The dividend yield is based on our history of dividend payouts. |

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| - Definition
The entire disclosure for compensation-related costs for equity-based compensation, which may include disclosure of policies, compensation plan details, allocation of equity compensation, incentive distributions, equity-based arrangements to obtain goods and services, deferred compensation arrangements, employee stock ownership plan details and employee stock purchase plan details.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 40 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6418621&loc=d3e17540-113929Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 505 -SubTopic 50 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=51659978&loc=d3e25284-112666Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5047-113901Reference 5: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 4 -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5444-113901Reference 6: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Staff Accounting Bulletin (SAB) -Number Topic 14
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