Capital Stock (Narrative) (Details) - USD ($)	1 Months Ended		12 Months Ended
	Jan. 31, 2018	Nov. 30, 2017	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Jan. 31, 2017	Jun. 30, 2014
Class of Stock [Line Items]
Stock repurchase program, authorized amount		$ 1,700,000,000.0	$ 8,500,000,000			$ 500,000,000	$ 1,500,000,000
Available to repurchase common stock			1,430,000,000
Amount expended on share repurchases in open market transactions			574,956,000	$ 28,689,000	$ 755,704,000
Accelerated stock repurchase program		$ 750,000,000	$ (750,000,000)
Total tax withholdings for share-based compensation (in shares)			974,501	830,155	679,694
Total tax withholdings for share-based compensation			$ 80,040,000	$ 66,638,000	$ 46,336,000
Preferred stock, shares authorized			5,000,000	5,000,000
Preferred stock, par value (in dollars per share)			$ 0.01	$ 0.01
Preferred stock, shares issued			0	0
Preferred stock, shares outstanding			0	0
Open Market Purchases
Class of Stock [Line Items]
Amount expended on share repurchases in open market transactions			$ 575,000,000	$ 28,700,000	$ 755,700,000
Number of shares repurchased			7,384,368	426,300	10,716,850
Average per share price on share repurchases in open market transactions (in dollars per share)			$ 77.86	$ 67.30	$ 70.52
ASR Counterparty
Class of Stock [Line Items]
Amount expended on share repurchases in open market transactions			$ 750,000,000
Number of shares repurchased			7,100,000.0
Shares repurchased as percentage of shares pursuant to accelerated share repurchase agreement (percent)			80.00%
Subsequent Event | ASR Counterparty
Class of Stock [Line Items]
Number of shares repurchased	1,371,495