Contracts Accounted for as Insurance - Net Expected Loss to be Expensed Insurance Contracts (Details) $ in Millions	Mar. 31, 2018 USD ($)
Insurance [Abstract]
2018 (April 1 – June 30)	$ 9
2018 (July 1 – September 30)	10
2018 (October 1 – December 31)	10
Subtotal 2018	29
2019	42
2020	39
2021	35
2022	32
2023-2027	126
2028-2032	77
2033-2037	35
After 2037	12
Net expected loss to be expensed	427
Future accretion	86
Total expected future loss and LAE	$ 513