Summary of Accounting Policies - Revision Of Previously Issued Financial Statements (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended			15 Months Ended	45 Months Ended	48 Months Ended
	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, 2016	Dec. 31, 2015	Dec. 31, 2014	Mar. 31, 2017	Dec. 31, 2016	Mar. 31, 2017
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Increase (decrease ) in Cost of sales	$ 1,794	$ 1,743	$ 1,816	$ 1,770	$ 1,687	$ 1,707	$ 1,757	$ 1,677	$ 7,123	$ 6,828	$ 6,989
Decrease in revenues									(8,599)	(8,181)	(8,381)
Adjustment
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Increase (decrease ) in Cost of sales									12	(17)	(36)
Adjustment | Lump Sum Payments Immediately Recognized, Should Have Been Deferred And Amortized
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Increase (decrease ) in Cost of sales													$ 28
Deferred charges	$ 23				$ 16				$ 23	16			$ 23
Adjustment | Correction To Revenues And Cost Of Sales Related For Agent Revenues [Member]
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Increase (decrease ) in Cost of sales										(28)	(39)
Decrease in revenues										$ 28	$ 39
Subsequent event | Adjustment | Lump Sum Payments Immediately Recognized, Should Have Been Deferred And Amortized
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Increase (decrease ) in Cost of sales														$ 34
Subsequent event | Adjustment | Accruals For Substrate Liabilities And Associated Cost Of Sales [Member]
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Increase (decrease ) in Cost of sales												$ 7