Derivatives and Hedging Activities - Credit Derivatives Sold and Held (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2019	Dec. 31, 2018
Credit Derivatives [Line Items]
Notional Amount	$ 64	$ 22
Average Term (Years)	0 years	0 years
Payment / Performance Risk	0.00%	0.00%
Other [Member]
Credit Derivatives [Line Items]
Notional Amount	$ 64	$ 22
Average Term (Years)	14 years 21 days	13 years 5 months 4 days
Payment / Performance Risk	2.98%	17.18%