Stockholders' Equity (Narrative) (Details) - USD ($)	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Jul. 25, 2017	Apr. 18, 2016
Equity, Class of Treasury Stock [Line Items]
Dividends paid, per share (USD per share)	$ 1.1	$ 1.1	$ 1.1	$ 1	$ 1	$ 1	$ 1	$ 0.9	$ 4.3	$ 3.9	$ 3.45
Stock repurchased during period, value									$ 682,000,000	$ 506,000,000	$ 163,000,000
Common Stock | Share Repurchase Program 2016
Equity, Class of Treasury Stock [Line Items]
Stock repurchase program, authorized amount													$ 1,000,000,000
Stock repurchased during period, shares (in shares)									4,010,000
Stock repurchased during period, value									$ 700,000,000
Stock repurchase program, remaining authorized repurchase amount	$ 0								$ 0
Common Stock | Share Repurchase Program 2017
Equity, Class of Treasury Stock [Line Items]
Stock repurchase program, authorized amount												$ 2,000,000,000
Stock repurchased during period, shares (in shares)									305,500
Stock repurchased during period, value									$ 50,000,000
Stock repurchase program, remaining authorized repurchase amount	$ 1,950,000,000								$ 1,950,000,000