Investments (Contractual maturity) (Details) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017
Investments [Abstract]
Due in one year or less, cost or amortized cost	$ 1,323	$ 1,135
Due after one year through five years, cost or amortized cost	8,277	8,165
Due after five years through ten years, cost or amortized cost	15,802	16,060
Due after ten years, cost or amortized cost	12,582	12,852
Total Amortized Cost Basis	37,984	38,212
Due in one year or less, estimated fair value	1,343	1,157
Due after one year through five years, estimated fair value	8,495	8,501
Due after five years through ten years, estimated fair value	16,093	16,718
Due after ten years, estimated fair value	14,321	15,108
Total Estimated Fair Value	$ 40,252	$ 41,484