Equity - Stock Repurchase Activities (Details) - USD ($) $ / shares in Units, shares in Millions		1 Months Ended	3 Months Ended				6 Months Ended	9 Months Ended
	Apr. 29, 2019	Feb. 28, 2018	Sep. 30, 2019	Jun. 30, 2019	Sep. 30, 2018	Jun. 30, 2018	Sep. 30, 2019	Sep. 30, 2019	Sep. 30, 2018	Jan. 31, 2018
Accelerated Share Repurchases [Line Items]
Payment to repurchase stock								$ 354,900,000	$ 755,500,000
Common stock received, aggregate price			$ 111,100,000		$ 151,300,000			354,900,000	$ 755,500,000
Stock Repurchase Program 2018
Accelerated Share Repurchases [Line Items]
Stock repurchase program, authorized amount			$ 1,700,000,000				$ 1,700,000,000	$ 1,700,000,000		$ 2,000,000,000.0
Stock repurchased and retired (in shares)			5.1		6.0			13.7	29.3
Average price per share (in dollars per share)			$ 24.90		$ 25.62			$ 25.45	$ 25.62
Stock repurchased (in shares)			2.1
Common stock received, aggregate price			$ 50,000,000.0		$ 0			$ 350,000,000.0	$ 750,000,000.0
Common stock repurchased, average cost per share (in dollars per share)			$ 23.63
Stock repurchase program, remaining authorized repurchase amount			$ 900,000,000.0				$ 900,000,000.0	$ 900,000,000.0
Accelerated Share Repurchase Program
Accelerated Share Repurchases [Line Items]
Stock repurchase program, authorized amount		$ 750,000,000.0
Payment to repurchase stock	$ 300,000,000.0	$ 750,000,000.0				$ 300,000,000.0
Stock repurchased and retired (in shares)				8.6				29.3
Average price per share (in dollars per share)	$ 27.94						$ 25.79	$ 25.62
Stock repurchased (in shares)			3.0				11.6
Common stock received, aggregate price						$ 240,000,000.0