Quarterly Results of Operations (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Quarterly Results of Operations (Unaudited) [Abstract]
Total revenues	$ 4,531	$ 4,264	$ 4,020	$ 3,609	$ 3,669	$ 3,511	$ 3,577	$ 3,500	$ 16,424	$ 14,257	$ 13,330
Total expenses	4,064	3,732	3,569	3,174	4,057	3,001	3,044	3,025	14,539	13,127	11,872
Net income (loss)	$ 399	$ 490	$ 385	$ 367	$ 816	$ 418	$ 411	$ 435	$ 1,641	$ 2,079	$ 1,192
Earnings (loss) per common share - basic:
Net income (loss) (in dollars per share)	$ 1.89	$ 2.27	$ 1.76	$ 1.68	$ 3.73	$ 1.89	$ 1.84	$ 1.93	$ 7.60	$ 9.36	$ 5.09
Earnings (loss) per common share - diluted:
Net income (loss) (in dollars per share)	$ 1.80	$ 2.24	$ 1.70	$ 1.64	$ 3.67	$ 1.87	$ 1.81	$ 1.89	$ 7.40	$ 9.22	$ 5.03