Quarterly Results of Operations (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Quarterly Results of Operations [Abstract]
Total revenues	$ 4,273	$ 4,227	$ 3,928	$ 3,764	$ 4,063	$ 4,039	$ 3,852	$ 3,404	$ 16,192	$ 15,358	$ 13,148
Total expenses	3,757	4,526	3,686	3,646	3,631	3,619	3,357	2,921	15,615	13,528	12,417
Net income (loss)	$ 460	$ (201)	$ 222	$ 133	$ 368	$ 378	$ 420	$ 407	$ 614	$ 1,573	$ 2,018