Fair Value Measurements (Details) - USD ($) $ in Millions	6 Months Ended	12 Months Ended
	Jun. 30, 2018	Dec. 31, 2017
Financial Instruments
Cash and cash equivalents	$ 873	$ 619
Short-term investments	$ 0	$ 100
Institutional and Other | Minimum
Financial Instruments
Fair value assumption, interest rate	4.60%	4.74%
Institutional and Other | Maximum
Financial Instruments
Fair value assumption, interest rate	7.45%	7.13%
Fair Value | Level 1
Financial Instruments
Cash and cash equivalents	$ 873	$ 619
Short-term investments	0	100
Fair Value | Level 2 | Retail
Financial Instruments
Long-term debt	1,772	1,783
Fair Value | Level 2 | Institutional
Financial Instruments
Long-term debt	546	522
Fair Value | Level 2 | Other
Financial Instruments
Long-term debt	187	194
Book Value
Financial Instruments
Cash and cash equivalents	873	619
Short-term investments	0	100
Book Value | Retail
Financial Instruments
Long-term debt	1,753	1,753
Book Value | Institutional
Financial Instruments
Long-term debt	534	534
Book Value | Other
Financial Instruments
Long-term debt	$ 188	$ 194