Reserve for Unpaid Losses and Loss Adjustment Expenses - Historical Loss Development Triangles, Group (Details) - Group Benefits claim in Millions, $ in Millions	Dec. 31, 2019 USD ($) claim	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)	Dec. 31, 2013 USD ($)	Dec. 31, 2012 USD ($)	Dec. 31, 2011 USD ($)
Claims Development [Line Items]
Cumulative Incurred for Accident Years Displayed in Triangles	$ 18,950
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance	12,648
Group long-term disability
Claims Development [Line Items]
Cumulative Incurred for Accident Years Displayed in Triangles	13,157
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance	7,316
Group long-term disability | 2011
Claims Development [Line Items]
Cumulative Incurred for Accident Years Displayed in Triangles	1,631	$ 1,638	$ 1,649	$ 1,660	$ 1,669	$ 1,659	$ 1,660	$ 1,761	$ 1,917
IBNR Reserves	$ 0
Claims Reported | claim	39,246
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance	$ 1,286	1,231	1,167	1,087	996	886	743	508	$ 118
Group long-term disability | 2012
Claims Development [Line Items]
Cumulative Incurred for Accident Years Displayed in Triangles	1,486	1,504	1,515	1,530	1,532	1,539	1,605	1,829
IBNR Reserves	$ 0
Claims Reported | claim	37,523
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance	$ 1,138	1,080	1,014	933	835	708	483	$ 108
Group long-term disability | 2013
Claims Development [Line Items]
Cumulative Incurred for Accident Years Displayed in Triangles	1,399	1,413	1,416	1,429	1,429	1,479	1,660
IBNR Reserves	$ 1
Claims Reported | claim	31,946
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance	$ 1,016	954	881	791	664	443	$ 102
Group long-term disability | 2014
Claims Development [Line Items]
Cumulative Incurred for Accident Years Displayed in Triangles	1,408	1,431	1,431	1,430	1,473	1,636
IBNR Reserves	$ 2
Claims Reported | claim	33,213
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance	$ 960	884	801	675	448	$ 103
Group long-term disability | 2015
Claims Development [Line Items]
Cumulative Incurred for Accident Years Displayed in Triangles	1,401	1,420	1,422	1,442	1,595
IBNR Reserves	$ 3
Claims Reported | claim	33,820
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance	$ 891	806	687	460	$ 108
Group long-term disability | 2016
Claims Development [Line Items]
Cumulative Incurred for Accident Years Displayed in Triangles	1,437	1,468	1,481	1,651
IBNR Reserves	$ 3
Claims Reported | claim	34,719
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance	$ 819	705	479	$ 112
Group long-term disability | 2017
Claims Development [Line Items]
Cumulative Incurred for Accident Years Displayed in Triangles	1,358	1,413	1,597
IBNR Reserves	$ 8
Claims Reported | claim	31,865
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance	$ 658	452	109
Group long-term disability | 2018
Claims Development [Line Items]
Cumulative Incurred for Accident Years Displayed in Triangles	1,387	1,647
IBNR Reserves	$ 37
Claims Reported | claim	28,551
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance	$ 447	105
Group long-term disability | 2019
Claims Development [Line Items]
Cumulative Incurred for Accident Years Displayed in Triangles	1,650
IBNR Reserves	$ 852
Claims Reported | claim	17,753
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance	$ 101
Group life and accident, excluding premium waiver
Claims Development [Line Items]
Cumulative Incurred for Accident Years Displayed in Triangles	5,793
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance	5,332
Group life and accident, excluding premium waiver | 2017
Claims Development [Line Items]
Cumulative Incurred for Accident Years Displayed in Triangles	1,951	1,953	1,999
IBNR Reserves	$ 4
Claims Reported | claim	45,139
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance	$ 1,945	1,929	$ 1,551
Group life and accident, excluding premium waiver | 2018
Claims Development [Line Items]
Cumulative Incurred for Accident Years Displayed in Triangles	1,940	1,952
IBNR Reserves	$ 18
Claims Reported | claim	52,027
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance	$ 1,916	$ 1,532
Group life and accident, excluding premium waiver | 2019
Claims Development [Line Items]
Cumulative Incurred for Accident Years Displayed in Triangles	1,902
IBNR Reserves	$ 373
Claims Reported | claim	45,825
Cumulative Paid Losses & Allocated Loss Adjustment Expenses, Net of Reinsurance	$ 1,471