Summary of Significant Accounting Policies - Accounting standards (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Income tax benefit	$ (802)	$ (877)	$ (1,111)
Unearned premiums	463	362	638
Equity securities	1,160	509
Limited partnership interests	6,740	5,814
Unrealized net capital gains and losses	319	433	(1,306)
Unrealized foreign currency translation adjustments	(47)	(10)	58
Unrecognized pension and other postretirement benefit cost	307	(104)	48
Accounting Standards Update 2017-09
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Income tax benefit	63
Accounting Standards Update 2014-09
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Unearned premiums	160
Deferred policy acquisition costs	125
Accounting Standards Update 2016-01
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Equity securities	1,160
EMA limited partnerships
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Limited partnership interests	5,410	4,530
EMA limited partnerships | Accounting Standards Update 2016-01
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Limited partnership interests	224
Accumulated other comprehensive income (loss)
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Reclassification of tax effects due to change in accounting principle	49	0	0
Unrealized net capital gains and losses	319	433	(1,306)
Unrealized foreign currency translation adjustments	(47)	(10)	58
Unrecognized pension and other postretirement benefit cost	307	(104)	48
Retained income
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Reclassification of tax effects due to change in accounting principle	(49)	$ 0	$ 0
Unrealized net capital gains and losses
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Reclassification of tax effects due to change in accounting principle	290
Unrealized foreign currency translation adjustments
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Reclassification of tax effects due to change in accounting principle	(6)
Unrecognized pension and other postretirement benefit cost
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Reclassification of tax effects due to change in accounting principle	$ (235)