Quarterly Results of Operations (Unaudited) (Details) - USD ($) $ / shares in Units, $ 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 Financial Information Disclosure [Abstract]
Total revenue	$ 4,274	$ 4,243	$ 4,193	$ 4,119	$ 4,664	$ 4,697	$ 4,883	$ 4,646	$ 16,829	$ 18,890	$ 19,883
Margin	578	561	583	581	664	566	673	615	2,303	2,518	1,810
Gain (loss) on sales of subsidiaries					(52)	37	0	0	0	(15)	0
Net income (loss)	$ 168	$ 175	$ 196	$ 198	$ 201	$ 197	$ 202	$ 107	$ 737	$ 707	$ (512)
Net income per share - Basic (in dollars per share)	$ 2.70	$ 2.81	$ 3.15	$ 3.19	$ 3.24	$ 3.22	$ 3.29	$ 1.79	$ 11.85	$ 11.57	$ (9.07)
Net income per share - Diluted (in dollars per share)	$ 2.67	$ 2.75	$ 3.06	$ 2.99	$ 3.01	$ 2.90	$ 3.02	$ 1.64	$ 11.47	$ 10.61	$ (9.07)