Equity - Narrative (Details)	1 Months Ended		3 Months Ended			6 Months Ended		12 Months Ended
	May 31, 2018 period shares	Feb. 28, 2018 USD ($) shares	Sep. 30, 2018	Jun. 30, 2018 USD ($) $ / shares	Jun. 30, 2017 USD ($) $ / shares	Jun. 30, 2018 USD ($) $ / shares shares	Jun. 30, 2017 USD ($) $ / shares	Dec. 31, 2017 USD ($)	Sep. 30, 2017 USD ($)
Class of Stock [Line Items]
Non-cash expense recognized				$ 32,000,000	$ 34,000,000	$ 61,000,000	$ 68,000,000
Vested stock options (in shares) | shares						1,829,220
Fair value of restricted stock						$ 195,000,000	$ 203,000,000
Authorized amount									$ 1,200,000,000.0
Repurchases of common stock (in shares) | shares						10,403,246
Repurchases of common stock						$ 759,000,000		$ 949,000,000
Remaining authorized repurchase amount (up to)				$ 441,000,000		$ 441,000,000
Dividend per share (in dollars per share) | $ / shares				$ 0.24	$ 0.20	$ 0.48	$ 0.40
Payments of ordinary dividends, common stock						$ 279,000,000	$ 239,000,000
Employee Stock Option
Class of Stock [Line Items]
Intrinsic value of shares				$ 10,000,000	$ 4,000,000	19,000,000	$ 7,000,000
Unrecognized compensation expense				12,000,000		$ 12,000,000
Cost not yet recognized, period for recognition						1 year 10 months 24 days
Employee Stock
Class of Stock [Line Items]
Number of shares authorized (up to) (in shares) | shares	25,000,000
Percentage in increment of eligible pay	1.00%
Percentage of maximum contribution of eligible pay	25.00%
Maximum number of shares per employee (in shares) | shares	25,000
Shares employees are eligible for in each offering period (in shares) | shares	1,250
Number of offering periods per year | period	2
Percentage of purchase price of common stock	85.00%
Performance Based Restricted Stock Units
Class of Stock [Line Items]
Unrecognized compensation expense		$ 42,000,000		31,000,000		$ 31,000,000
Cost not yet recognized, period for recognition		3 years
Shares reserved for future issuance (in shares) | shares		1,303,151
Vested stock options (in shares) | shares		651,576
Stock-based compensation				7,000,000		11,000,000
Amount of non-cash compensation remaining in fiscal period				12,000,000		12,000,000
Time Based And Performance Based Restricted Stock
Class of Stock [Line Items]
Unrecognized compensation expense				$ 181,000,000		$ 181,000,000
Cost not yet recognized, period for recognition						1 year 7 months 6 days
Maximum | Performance Based Restricted Stock Units
Class of Stock [Line Items]
Unrecognized compensation expense		$ 84,000,000
Scenario, Forecast | Employee Stock
Class of Stock [Line Items]
Discount from market price			15.00%