v3.8.0.1
Investments (Credit Quality Indicators) (Details) - USD ($)
$ in Millions
Dec. 31, 2017
Dec. 31, 2016
Commercial Mortgage Loans    
Financing Receivable, Recorded Investment [Line Items]    
Financing Receivable, Gross $ 52,084 $ 49,027
Agricultural property loans    
Financing Receivable, Recorded Investment [Line Items]    
Financing Receivable, Gross 3,203 2,958
Commercial mortgage and agricultural property loans    
Financing Receivable, Recorded Investment [Line Items]    
Financing Receivable, Gross 55,287 51,985
0%-59.99% | Commercial Mortgage Loans    
Financing Receivable, Recorded Investment [Line Items]    
Financing Receivable, Gross 30,972 29,203
0%-59.99% | Agricultural property loans    
Financing Receivable, Recorded Investment [Line Items]    
Financing Receivable, Gross 3,163 2,934
0%-59.99% | Commercial mortgage and agricultural property loans    
Financing Receivable, Recorded Investment [Line Items]    
Financing Receivable, Gross 34,135 32,137
60%-69.99% | Commercial Mortgage Loans    
Financing Receivable, Recorded Investment [Line Items]    
Financing Receivable, Gross 14,309 13,124
60%-69.99% | Agricultural property loans    
Financing Receivable, Recorded Investment [Line Items]    
Financing Receivable, Gross 40 24
60%-69.99% | Commercial mortgage and agricultural property loans    
Financing Receivable, Recorded Investment [Line Items]    
Financing Receivable, Gross 14,349 13,148
70%-79.99% | Commercial Mortgage Loans    
Financing Receivable, Recorded Investment [Line Items]    
Financing Receivable, Gross 6,537 6,133
70%-79.99% | Agricultural property loans    
Financing Receivable, Recorded Investment [Line Items]    
Financing Receivable, Gross 0 0
70%-79.99% | Commercial mortgage and agricultural property loans    
Financing Receivable, Recorded Investment [Line Items]    
Financing Receivable, Gross 6,537 6,133
80% or greater | Commercial Mortgage Loans    
Financing Receivable, Recorded Investment [Line Items]    
Financing Receivable, Gross 266 567
80% or greater | Agricultural property loans    
Financing Receivable, Recorded Investment [Line Items]    
Financing Receivable, Gross 0 0
80% or greater | Commercial mortgage and agricultural property loans    
Financing Receivable, Recorded Investment [Line Items]    
Financing Receivable, Gross 266 567
1.2X | Commercial Mortgage Loans    
Financing Receivable, Recorded Investment [Line Items]    
Financing Receivable, Gross 49,827 46,495
1.2X | Agricultural property loans    
Financing Receivable, Recorded Investment [Line Items]    
Financing Receivable, Gross 3,028 2,827
1.2X | Commercial mortgage and agricultural property loans    
Financing Receivable, Recorded Investment [Line Items]    
Financing Receivable, Gross 52,855 49,322
1.2X | 0%-59.99% | Commercial Mortgage Loans    
Financing Receivable, Recorded Investment [Line Items]    
Financing Receivable, Gross 30,082 28,131
1.2X | 0%-59.99% | Agricultural property loans    
Financing Receivable, Recorded Investment [Line Items]    
Financing Receivable, Gross 2,988 2,803
1.2X | 0%-59.99% | Commercial mortgage and agricultural property loans    
Financing Receivable, Recorded Investment [Line Items]    
Financing Receivable, Gross 33,070 30,934
1.2X | 60%-69.99% | Commercial Mortgage Loans    
Financing Receivable, Recorded Investment [Line Items]    
Financing Receivable, Gross 13,658 12,608
1.2X | 60%-69.99% | Agricultural property loans    
Financing Receivable, Recorded Investment [Line Items]    
Financing Receivable, Gross 40 24
1.2X | 60%-69.99% | Commercial mortgage and agricultural property loans    
Financing Receivable, Recorded Investment [Line Items]    
Financing Receivable, Gross 13,698 12,632
1.2X | 70%-79.99% | Commercial Mortgage Loans    
Financing Receivable, Recorded Investment [Line Items]    
Financing Receivable, Gross 5,994 5,383
1.2X | 70%-79.99% | Agricultural property loans    
Financing Receivable, Recorded Investment [Line Items]    
Financing Receivable, Gross 0 0
1.2X | 70%-79.99% | Commercial mortgage and agricultural property loans    
Financing Receivable, Recorded Investment [Line Items]    
Financing Receivable, Gross 5,994 5,383
1.2X | 80% or greater | Commercial Mortgage Loans    
Financing Receivable, Recorded Investment [Line Items]    
Financing Receivable, Gross 93 373
1.2X | 80% or greater | Agricultural property loans    
Financing Receivable, Recorded Investment [Line Items]    
Financing Receivable, Gross 0 0
1.2X | 80% or greater | Commercial mortgage and agricultural property loans    
Financing Receivable, Recorded Investment [Line Items]    
Financing Receivable, Gross 93 373
1.0X to 1.2X | Commercial Mortgage Loans    
Financing Receivable, Recorded Investment [Line Items]    
Financing Receivable, Gross 1,737 1,603
1.0X to 1.2X | Agricultural property loans    
Financing Receivable, Recorded Investment [Line Items]    
Financing Receivable, Gross 170 114
1.0X to 1.2X | Commercial mortgage and agricultural property loans    
Financing Receivable, Recorded Investment [Line Items]    
Financing Receivable, Gross 1,907 1,717
1.0X to 1.2X | 0%-59.99% | Commercial Mortgage Loans    
Financing Receivable, Recorded Investment [Line Items]    
Financing Receivable, Gross 639 446
1.0X to 1.2X | 0%-59.99% | Agricultural property loans    
Financing Receivable, Recorded Investment [Line Items]    
Financing Receivable, Gross 170 114
1.0X to 1.2X | 0%-59.99% | Commercial mortgage and agricultural property loans    
Financing Receivable, Recorded Investment [Line Items]    
Financing Receivable, Gross 809 560
1.0X to 1.2X | 60%-69.99% | Commercial Mortgage Loans    
Financing Receivable, Recorded Investment [Line Items]    
Financing Receivable, Gross 530 401
1.0X to 1.2X | 60%-69.99% | Agricultural property loans    
Financing Receivable, Recorded Investment [Line Items]    
Financing Receivable, Gross 0 0
1.0X to 1.2X | 60%-69.99% | Commercial mortgage and agricultural property loans    
Financing Receivable, Recorded Investment [Line Items]    
Financing Receivable, Gross 530 401
1.0X to 1.2X | 70%-79.99% | Commercial Mortgage Loans    
Financing Receivable, Recorded Investment [Line Items]    
Financing Receivable, Gross 514 694
1.0X to 1.2X | 70%-79.99% | Agricultural property loans    
Financing Receivable, Recorded Investment [Line Items]    
Financing Receivable, Gross 0 0
1.0X to 1.2X | 70%-79.99% | Commercial mortgage and agricultural property loans    
Financing Receivable, Recorded Investment [Line Items]    
Financing Receivable, Gross 514 694
1.0X to 1.2X | 80% or greater | Commercial Mortgage Loans    
Financing Receivable, Recorded Investment [Line Items]    
Financing Receivable, Gross 54 62
1.0X to 1.2X | 80% or greater | Agricultural property loans    
Financing Receivable, Recorded Investment [Line Items]    
Financing Receivable, Gross 0 0
1.0X to 1.2X | 80% or greater | Commercial mortgage and agricultural property loans    
Financing Receivable, Recorded Investment [Line Items]    
Financing Receivable, Gross 54 62
1.0X | Commercial Mortgage Loans    
Financing Receivable, Recorded Investment [Line Items]    
Financing Receivable, Gross 520 929
1.0X | Agricultural property loans    
Financing Receivable, Recorded Investment [Line Items]    
Financing Receivable, Gross 5 17
1.0X | Commercial mortgage and agricultural property loans    
Financing Receivable, Recorded Investment [Line Items]    
Financing Receivable, Gross 525 946
1.0X | 0%-59.99% | Commercial Mortgage Loans    
Financing Receivable, Recorded Investment [Line Items]    
Financing Receivable, Gross 251 626
1.0X | 0%-59.99% | Agricultural property loans    
Financing Receivable, Recorded Investment [Line Items]    
Financing Receivable, Gross 5 17
1.0X | 0%-59.99% | Commercial mortgage and agricultural property loans    
Financing Receivable, Recorded Investment [Line Items]    
Financing Receivable, Gross 256 643
1.0X | 60%-69.99% | Commercial Mortgage Loans    
Financing Receivable, Recorded Investment [Line Items]    
Financing Receivable, Gross 121 115
1.0X | 60%-69.99% | Agricultural property loans    
Financing Receivable, Recorded Investment [Line Items]    
Financing Receivable, Gross 0 0
1.0X | 60%-69.99% | Commercial mortgage and agricultural property loans    
Financing Receivable, Recorded Investment [Line Items]    
Financing Receivable, Gross 121 115
1.0X | 70%-79.99% | Commercial Mortgage Loans    
Financing Receivable, Recorded Investment [Line Items]    
Financing Receivable, Gross 29 56
1.0X | 70%-79.99% | Agricultural property loans    
Financing Receivable, Recorded Investment [Line Items]    
Financing Receivable, Gross 0 0
1.0X | 70%-79.99% | Commercial mortgage and agricultural property loans    
Financing Receivable, Recorded Investment [Line Items]    
Financing Receivable, Gross 29 56
1.0X | 80% or greater | Commercial Mortgage Loans    
Financing Receivable, Recorded Investment [Line Items]    
Financing Receivable, Gross 119 132
1.0X | 80% or greater | Agricultural property loans    
Financing Receivable, Recorded Investment [Line Items]    
Financing Receivable, Gross 0 0
1.0X | 80% or greater | Commercial mortgage and agricultural property loans    
Financing Receivable, Recorded Investment [Line Items]    
Financing Receivable, Gross $ 119 $ 132