Mergers and Acquisitions - Transaction-related Costs (Details) - USD ($) $ 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
Before-tax [Abstract]
Interest expense on newly issued debt									$ 227	$ 0	$ 0
Net investment income on debt proceeds									(123)	0	0
Charitable contributions									200	0	0
Legal and advisory fees									204	36	96
Bridge facility fees									140	0	0
All other transaction-related costs									204	90	70
Transaction-related costs, net									852	126	166
After-tax [Abstract]
Interest expense on newly issued debt, after-tax									179	0	0
Net investment income on debt proceeds, after-tax									(97)	0	0
Charitable contributions, after-tax									158	0	0
Legal and advisory fees, after-tax									185	23	95
Bridge facility fees, after-tax									111	0	0
All other transaction-related costs, after-tax									133	69	52
Tax (benefit) - previously non-deductible costs										(59)
Transaction-related costs, net - after-tax	$ 402	$ 108	$ 109	$ 50	$ 25	$ 6	$ (47)	$ 49	$ 669	$ 33	$ 147