Business, Basis of Presentation and Summary of Significant Accounting Policies - New Accounting Pronouncements (Details) - USD ($) $ in Millions	Jan. 01, 2018	Dec. 31, 2017	Mar. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Premiums and Other Receivables, Net		$ 18,423		$ 15,445
Payables For Collateral Under Securities Loaned And Other Transactions		25,723		25,873
Derivative assets		8,551		12,139
Derivative liabilities		4,339		6,921
Accrued Investment Income Receivable		3,524		3,308
Other Liabilities		23,982		23,735
Retained Earnings (Accumulated Deficit)		26,527		34,683	$ 35,672	$ 32,110
Accumulated Other Comprehensive Income (Loss), Net of Tax		7,427		$ 5,366	$ 4,767	$ 10,714
CME update impact
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Premiums and Other Receivables, Net			$ 991
Payables For Collateral Under Securities Loaned And Other Transactions			816
Derivative assets			1,800
Derivative liabilities			2,000
Accrued Investment Income Receivable			101
Other Liabilities			$ 14
Adjustments for New Accounting Pronouncement | Subsequent Event
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Retained Earnings (Accumulated Deficit)	$ 1,200
Accumulated Other Comprehensive Income (Loss), Net of Tax	$ 1,200
Accounting Standards Update 2016-01
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
New Accounting Pronouncement or Change in Accounting Principle, Effect of Change on Equity		$ 280