General (Details) - USD ($) $ in Millions	6 Months Ended
	Jun. 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	13,824			$ 13,473
Deferred policy acquisition costs	4,533			4,191
Assets	113,369			112,422
Liabilities	$ 90,247			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
Scenario, Forecast [Member] | Accounting Standards Update 2017-13
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Assets		$ 500
Liabilities		500
Scenario, Forecast [Member] | 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