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, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Unpaid losses and loss adjustment expenses	$ 32,287	$ 28,317
Group Insurance Policies
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Cumulative Incurred for Accident Years Displayed in Triangles	16,350
Cumulative Paid for Accident Years Displayed in Triangles	(10,488)
Unpaid for Accident Years not Displayed in Triangles	3,796
Unpaid Unallocated Loss Adjustment Expenses, Net of Reinsurance	181
Discount	(1,536)	(1,303)	$ (1,382)
Unpaid Losses and Loss Adjustment Expenses, Net of Reinsurance	8,303	5,564	5,671	$ 5,804
Reinsurance and Other Recoverables	209	208	218	209
Unpaid losses and loss adjustment expenses	8,512	$ 5,772	$ 5,889	$ 6,013
Group Insurance Policies | Group long-term disability
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Cumulative Incurred for Accident Years Displayed in Triangles	10,511
Cumulative Paid for Accident Years Displayed in Triangles	(5,138)
Unpaid for Accident Years not Displayed in Triangles	2,651
Unpaid Unallocated Loss Adjustment Expenses, Net of Reinsurance	166
Discount	(1,383)
Unpaid Losses and Loss Adjustment Expenses, Net of Reinsurance	6,807
Reinsurance and Other Recoverables	207
Unpaid losses and loss adjustment expenses	7,014
Group Insurance Policies | 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	5,839
Cumulative Paid for Accident Years Displayed in Triangles	(5,350)
Unpaid for Accident Years not Displayed in Triangles	138
Unpaid Unallocated Loss Adjustment Expenses, Net of Reinsurance	3
Discount	(21)
Unpaid Losses and Loss Adjustment Expenses, Net of Reinsurance	609
Reinsurance and Other Recoverables	1
Unpaid losses and loss adjustment expenses	610
Group Insurance Policies | Group short-term disability
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Unpaid for Accident Years not Displayed in Triangles	99
Unpaid Unallocated Loss Adjustment Expenses, Net of Reinsurance	5
Discount	0
Unpaid Losses and Loss Adjustment Expenses, Net of Reinsurance	104
Reinsurance and Other Recoverables	0
Unpaid losses and loss adjustment expenses	104
Group Insurance Policies | Group life premium waiver
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Unpaid for Accident Years not Displayed in Triangles	869
Unpaid Unallocated Loss Adjustment Expenses, Net of Reinsurance	7
Discount	(132)
Unpaid Losses and Loss Adjustment Expenses, Net of Reinsurance	744
Reinsurance and Other Recoverables	1
Unpaid losses and loss adjustment expenses	745
Group Insurance Policies | Group supplemental health
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Unpaid for Accident Years not Displayed in Triangles	39
Unpaid Unallocated Loss Adjustment Expenses, Net of Reinsurance	0
Discount	0
Unpaid Losses and Loss Adjustment Expenses, Net of Reinsurance	39
Reinsurance and Other Recoverables	0
Unpaid losses and loss adjustment expenses	$ 39