Reserve for Unpaid Losses and Loss Adjustment Expenses - Reconciliation of Loss Development to Liability for Unpaid Losses and Loss Adjustment Expenses, Group (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Unpaid losses and loss adjustment expenses	$ 36,517	$ 33,029
Group Benefits
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Cumulative Incurred for Accident Years Displayed in Triangles	18,950
Cumulative Paid for Accident Years Displayed in Triangles	(12,648)
Unpaid for Accident Years not Displayed in Triangles	2,918
Unpaid Unallocated Loss Adjustment Expenses, Net of Reinsurance	190
Discount	(1,401)	(1,505)	$ (1,536)
Unpaid Losses and Loss Adjustment Expenses, Net of Reinsurance	8,009	8,206	8,303	$ 5,564
Reinsurance and Other Recoverables	(247)	(239)	(209)	(208)
Unpaid losses and loss adjustment expenses	8,256	$ 8,445	$ 8,512	$ 5,772
Group Benefits | Group long-term disability
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Cumulative Incurred for Accident Years Displayed in Triangles	13,157
Cumulative Paid for Accident Years Displayed in Triangles	(7,316)
Unpaid for Accident Years not Displayed in Triangles	1,874
Unpaid Unallocated Loss Adjustment Expenses, Net of Reinsurance	173
Discount	(1,272)
Unpaid Losses and Loss Adjustment Expenses, Net of Reinsurance	6,616
Reinsurance and Other Recoverables	(236)
Unpaid losses and loss adjustment expenses	6,852
Group Benefits | Group life and accident, excluding premium waiver
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Cumulative Incurred for Accident Years Displayed in Triangles	5,793
Cumulative Paid for Accident Years Displayed in Triangles	(5,332)
Unpaid for Accident Years not Displayed in Triangles	134
Unpaid Unallocated Loss Adjustment Expenses, Net of Reinsurance	3
Discount	(18)
Unpaid Losses and Loss Adjustment Expenses, Net of Reinsurance	580
Reinsurance and Other Recoverables	(1)
Unpaid losses and loss adjustment expenses	581
Group Benefits | Group short-term disability
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Unpaid for Accident Years not Displayed in Triangles	114
Unpaid Unallocated Loss Adjustment Expenses, Net of Reinsurance	4
Discount	0
Unpaid Losses and Loss Adjustment Expenses, Net of Reinsurance	118
Reinsurance and Other Recoverables	0
Unpaid losses and loss adjustment expenses	118
Group Benefits | Group life premium waiver
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Unpaid for Accident Years not Displayed in Triangles	758
Unpaid Unallocated Loss Adjustment Expenses, Net of Reinsurance	10
Discount	(111)
Unpaid Losses and Loss Adjustment Expenses, Net of Reinsurance	657
Reinsurance and Other Recoverables	(2)
Unpaid losses and loss adjustment expenses	659
Group Benefits | Group supplemental health
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Unpaid for Accident Years not Displayed in Triangles	38
Unpaid Unallocated Loss Adjustment Expenses, Net of Reinsurance	0
Discount	0
Unpaid Losses and Loss Adjustment Expenses, Net of Reinsurance	38
Reinsurance and Other Recoverables	(8)
Unpaid losses and loss adjustment expenses	$ 46