Business, Basis of Presentation and Summary of Significant Accounting Policies - New Accounting Pronouncements (Details) - USD ($) $ in Millions	Dec. 31, 2018	Mar. 31, 2018	Jan. 01, 2018	Dec. 31, 2017	Mar. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Premiums and Other Receivables, Net	$ 19,644			$ 18,423
Payables For Collateral Under Securities Loaned And Other Transactions	24,794			25,723
Derivative assets	8,969			8,551
Derivative liabilities	3,930			4,339
Accrued Investment Income Receivable	3,582			3,524
Other Liabilities	22,964			23,982
Retained Earnings (Accumulated Deficit)	28,926			26,527
Accumulated Other Comprehensive Income (Loss), Net of Tax	1,722			7,427		$ 5,366	$ 4,767
Equity securities	1,440			2,513
Debt Securities, Trading, and Equity Securities, FV-NI	$ 12,616			16,745
Accounting Standards Update 2018-02
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]
Retained Earnings (Accumulated Deficit)			328
Equity securities			1,700
Debt Securities, Trading, and Equity Securities, FV-NI				$ 16,000
LCH update impact
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Premiums and Other Receivables, Net		$ (184)
Payables For Collateral Under Securities Loaned And Other Transactions		(365)
Derivative assets		(369)
Derivative liabilities		(203)
Accrued Investment Income Receivable		(14)
Other Liabilities		$ 1
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)
Maximum | Accounting Standards Update 2017-12
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Retained Earnings (Accumulated Deficit)			$ (250)