Investments - Fair Values (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Amortized cost, gross unrealized gains and losses and fair value for fixed income securities
Amortized cost	$ 20,217	$ 21,057
Gross unrealized, gains	1,549	732
Gross unrealized, losses	(41)	(389)
Fair value	21,725	21,400
U.S. government and agencies
Amortized cost, gross unrealized gains and losses and fair value for fixed income securities
Amortized cost	848	740
Gross unrealized, gains	34	33
Gross unrealized, losses	0	0
Fair value	882	773
Municipal
Amortized cost, gross unrealized gains and losses and fair value for fixed income securities
Amortized cost	1,483	1,997
Gross unrealized, gains	279	202
Gross unrealized, losses	(7)	(4)
Fair value	1,755	2,195
Corporate
Amortized cost, gross unrealized gains and losses and fair value for fixed income securities
Amortized cost	17,301	17,534
Gross unrealized, gains	1,170	434
Gross unrealized, losses	(30)	(381)
Fair value	18,441	17,587
Foreign government
Amortized cost, gross unrealized gains and losses and fair value for fixed income securities
Amortized cost	142	170
Gross unrealized, gains	7	9
Gross unrealized, losses	0	0
Fair value	149	179
ABS
Amortized cost, gross unrealized gains and losses and fair value for fixed income securities
Amortized cost	316	429
Gross unrealized, gains	4	3
Gross unrealized, losses	(3)	(3)
Fair value	317	429
MBS
Amortized cost, gross unrealized gains and losses and fair value for fixed income securities
Amortized cost	127	187
Gross unrealized, gains	55	51
Gross unrealized, losses	(1)	(1)
Fair value	$ 181	$ 237