General (Details) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2018	Dec. 31, 2018	Jan. 01, 2018	Dec. 31, 2017	Dec. 31, 2016
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Equity securities, unrealized gain		$ 910
Unearned premiums	$ 14,408	14,408			$ 13,473
Deferred policy acquisition costs	4,667	4,667			4,191
Tax Legislation expense	31
Assets	114,490	114,490			112,422
Liabilities	$ 90,857	$ 90,857			89,871
Difference between Revenue Guidance in Effect before and after Topic 606 | Accounting Standards Update 2014-09
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Unearned premiums				$ 160
Deferred policy acquisition costs				$ 160
Retained income
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative effect of change in accounting principle					1,088	$ 0
Retained income | Accounting Standards Update 2016-01
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative effect of change in accounting principle					1,160
Limited partnership interests | Accounting Standards Update 2016-01
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative effect of change in accounting principle					224
Limited partnership interests | Retained income | Accounting Standards Update 2016-01
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative effect of change in accounting principle					$ 177
Forecast | Accounting Standards Update 2017-13
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Assets			$ 525
Liabilities			525
Forecast | Retained income | Accounting Standards Update 2017-13
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative effect of change in accounting principle			$ 20