Fair Value Measurements (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2018	Dec. 31, 2017
Financial Instruments
Cash and cash equivalents	$ 509	$ 352
Short-term investments	$ 0	$ 50
Institutional and Other | Minimum
Financial Instruments
Fair value assumption, interest rate	4.58%	4.74%
Institutional and Other | Maximum
Financial Instruments
Fair value assumption, interest rate	6.34%	7.13%
Fair Value | Level 1
Financial Instruments
Cash and cash equivalents	$ 509	$ 352
Short-term investments	0	50
Fair Value | Level 2 | Retail
Financial Instruments
Long-term debt	944	939
Fair Value | Level 2 | Institutional
Financial Instruments
Long-term debt	555	522
Fair Value | Level 2 | Other
Financial Instruments
Long-term debt	188	191
Book Value
Financial Instruments
Cash and cash equivalents	509	352
Short-term investments	0	50
Book Value | Retail
Financial Instruments
Long-term debt	917	917
Book Value | Institutional
Financial Instruments
Long-term debt	534	534
Book Value | Other
Financial Instruments
Long-term debt	$ 188	$ 191