Derivatives and Hedging Activities - Credit Risk Contingent Feature (Details) - USD ($) $ in Millions	Sep. 30, 2019	Dec. 31, 2018
Credit Derivatives [Line Items]
Additional collateral aggregate fair value	$ 5
Standard & Poor's, A- Rating [Member] | One Rating Downgrade [Member]
Credit Derivatives [Line Items]
Additional collateral aggregate fair value	0	$ 2
Standard & Poor's, A- Rating [Member] | Two Rating Downgrade [Member]
Credit Derivatives [Line Items]
Additional collateral aggregate fair value	0	2
Standard & Poor's, A- Rating [Member] | Three Rating Downgrade [Member]
Credit Derivatives [Line Items]
Additional collateral aggregate fair value	0	2
Moody's, A3 Rating [Member] | One Rating Downgrade [Member]
Credit Derivatives [Line Items]
Additional collateral aggregate fair value	0	2
Moody's, A3 Rating [Member] | Two Rating Downgrade [Member]
Credit Derivatives [Line Items]
Additional collateral aggregate fair value	0	2
Moody's, A3 Rating [Member] | Three Rating Downgrade [Member]
Credit Derivatives [Line Items]
Additional collateral aggregate fair value	$ 0	$ 2