Changes in Capital Structure - Narrative (Details) - USD ($)	1 Months Ended			2 Months Ended	3 Months Ended
	Oct. 31, 2019	Apr. 30, 2019	Mar. 31, 2019	Feb. 28, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2018
Capital Structure
Common stock, shares authorized (in shares)					500,000,000						500,000,000
Stock repurchase program, authorized amount				$ 400,000,000
Shares repurchased, value					$ 234,000,000	$ 305,000,000	$ 749,000,000	$ 500,000,000	$ 407,000,000	$ 93,000,000
Employee Stock Purchase Plan, minimum percentage of eligible compensation to purchase shares			1.00%
Employee Stock Purchase Plan, maximum percentage of eligible compensation			10.00%
Employee Stock Purchase Plan, exercise price as percentage of fair value			95.00%
Subsequent Event
Capital Structure
Shares issued to employee accounts (in shares)	46,128
Additional Paid-in Capital
Capital Structure
Shares repurchased, value						$ (10,000,000)	$ 10,000,000	$ 37,000,000	$ 11,000,000
Accelerated Share Repurchase Program
Capital Structure
Shares repurchased (in shares)		351,768	9,086,903
Shares repurchased, value			$ 390,000,000
Average price paid per share (in usd per share)		$ 42.38
Accelerated Share Repurchase Program | Additional Paid-in Capital
Capital Structure
Remaining amount recorded in additional paid in capital		$ 10,000,000
2018 Plan
Capital Structure
Average price paid per share (in usd per share)				$ 40,610,000
2019 Plan
Capital Structure
Stock repurchase program, authorized amount				$ 1,000,000,000