Stockholders' Equity - Repurchase of Common Stock (Details) - USD ($)	1 Months Ended	3 Months Ended		6 Months Ended
	Feb. 28, 2018	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	May 08, 2018	Dec. 31, 2017	Nov. 30, 2017	Jul. 31, 2015
Equity, Class of Treasury Stock [Line Items]
Value of shares repurchased				$ 401,800,000	$ 305,300,000
Shares acquired under program (in shares)		71,673,973		71,673,973			61,485,423
July 2015 Repurchase Program
Equity, Class of Treasury Stock [Line Items]
Stock repurchase program, authorized amount									$ 1,500,000,000
Stock repurchase program, authorized increase			$ 1,500,000,000.0		$ 1,500,000,000.0	$ 1,000,000,000.0
Number of shares repurchased (in shares)		1,780,578	3,536,308	10,575,532	3,536,308
Value of shares repurchased		$ 77,100,000	$ 155,300,000	$ 481,600,000	$ 155,300,000
Accelerated Share Repurchase Agreement
Equity, Class of Treasury Stock [Line Items]
Stock repurchase program, authorized amount								$ 400,000,000
Shares acquired under program (in shares)	8,308,692						7,089,056
Shares acquired, average price per share (in usd per share)	$ 48.14
Share Trading Plan | Common Stock Repurchase Set One
Equity, Class of Treasury Stock [Line Items]
Number of shares repurchased (in shares)		1,780,578		9,355,896
Value of shares repurchased		$ 401,600,000
Shares acquired, average price per share (in usd per share)		$ 43.29		$ 42.93