Fair Value Measurements - Recurring Reconciliation (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Reconciliation of items measured at fair value on a recurring basis that used significant unobservable inputs (Level 3)
Balance at the beginning of the period	$ 30	$ 20	$ 30	$ 20
Total gains or losses included in earnings	0	0	0	0
Total gains or losses included in other comprehensive income	0	0	0	0
Purchases and issuances	0	0	0	0
Sales and settlements	0	0	0	0
Transfers in and/or out of Level 3	0	0	0	0
Balance at the end of the period	$ 30	$ 20	$ 30	$ 20