Stockholders' Equity - Repurchase of Common Stock (Details) - USD ($)	1 Months Ended	3 Months Ended		9 Months Ended
	Feb. 28, 2018	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	May 31, 2018	Dec. 31, 2017	Nov. 30, 2017	Mar. 31, 2017	Jul. 31, 2015
Equity, Class of Treasury Stock [Line Items]
Stock repurchase program, authorized amount			$ 400,000,000.0		$ 400,000,000.0
Value of shares repurchased		$ 121,500,000	$ 452,000,000	$ 523,300,000	$ 757,300,000
Shares acquired under program (in shares)		74,672,913		74,672,913			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						$ 1,000,000,000.0			$ 1,500,000,000.0
Number of shares repurchased (in shares)		2,959,638	1,661,782	13,535,170	5,198,090
Value of shares repurchased		$ 121,500,000	$ 72,400,000	$ 603,100,000	$ 227,700,000
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)		2,959,638		12,315,534
Value of shares repurchased		$ 523,100,000
Shares acquired, average price per share (in usd per share)		$ 41.04		$ 42.47