Investments (Credit Quality of Commercial Mortgage Loans) (Details) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans Receivable, Gross, Commercial, Mortgage	$ 47,460	$ 44,375
Loans Receivable Commercial Mortgage Percentage	100.00%	100.00%
Estimated Fair Value Commercial Mortgage	$ 47,406	$ 45,084
Estimated Fair Value Commercial Mortgage Percentage	100.00%	100.00%
Debt Service Coverage Ratio Range Two [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans Receivable, Gross, Commercial, Mortgage	$ 1,525	$ 1,959
Debt Service Coverage Ratio Range Three [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans Receivable, Gross, Commercial, Mortgage	142	524
Debt Service Coverage Ratio Range One [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans Receivable, Gross, Commercial, Mortgage	45,793	41,892
Mortgage Loans By Credit Quality Indicator Range Two [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans Receivable, Gross, Commercial, Mortgage	$ 5,182	$ 4,400
Loans Receivable Commercial Mortgage Percentage	10.90%	9.90%
Estimated Fair Value Commercial Mortgage	$ 5,152	$ 4,408
Estimated Fair Value Commercial Mortgage Percentage	10.90%	9.80%
Mortgage Loans By Credit Quality Indicator Range Two [Member] | Debt Service Coverage Ratio Range Two [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans Receivable, Gross, Commercial, Mortgage	$ 381	$ 98
Mortgage Loans By Credit Quality Indicator Range Two [Member] | Debt Service Coverage Ratio Range Three [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans Receivable, Gross, Commercial, Mortgage	69	119
Mortgage Loans By Credit Quality Indicator Range Two [Member] | Debt Service Coverage Ratio Range One [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans Receivable, Gross, Commercial, Mortgage	4,732	4,183
Mortgage Loans By Credit Quality Indicator Range Three [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans Receivable, Gross, Commercial, Mortgage	$ 653	$ 502
Loans Receivable Commercial Mortgage Percentage	1.40%	1.10%
Estimated Fair Value Commercial Mortgage	$ 621	$ 476
Estimated Fair Value Commercial Mortgage Percentage	1.30%	1.00%
Mortgage Loans By Credit Quality Indicator Range Three [Member] | Debt Service Coverage Ratio Range Two [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans Receivable, Gross, Commercial, Mortgage	$ 210	$ 210
Mortgage Loans By Credit Quality Indicator Range Three [Member] | Debt Service Coverage Ratio Range Three [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans Receivable, Gross, Commercial, Mortgage	56	57
Mortgage Loans By Credit Quality Indicator Range Three [Member] | Debt Service Coverage Ratio Range One [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans Receivable, Gross, Commercial, Mortgage	387	235
Mortgage Loans By Credit Quality Indicator Range Four [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans Receivable, Gross, Commercial, Mortgage	$ 655	$ 716
Loans Receivable Commercial Mortgage Percentage	1.40%	1.60%
Estimated Fair Value Commercial Mortgage	$ 619	$ 672
Estimated Fair Value Commercial Mortgage Percentage	1.30%	1.50%
Mortgage Loans By Credit Quality Indicator Range Four [Member] | Debt Service Coverage Ratio Range Two [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans Receivable, Gross, Commercial, Mortgage	$ 176	$ 168
Mortgage Loans By Credit Quality Indicator Range Four [Member] | Debt Service Coverage Ratio Range Three [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans Receivable, Gross, Commercial, Mortgage	0	147
Mortgage Loans By Credit Quality Indicator Range Four [Member] | Debt Service Coverage Ratio Range One [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans Receivable, Gross, Commercial, Mortgage	479	401
Mortgage Loans By Credit Quality Indicator Range One [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans Receivable, Gross, Commercial, Mortgage	$ 40,970	$ 38,757
Loans Receivable Commercial Mortgage Percentage	86.30%	87.40%
Estimated Fair Value Commercial Mortgage	$ 41,014	$ 39,528
Estimated Fair Value Commercial Mortgage Percentage	86.50%	87.70%
Mortgage Loans By Credit Quality Indicator Range One [Member] | Debt Service Coverage Ratio Range Two [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans Receivable, Gross, Commercial, Mortgage	$ 758	$ 1,483
Mortgage Loans By Credit Quality Indicator Range One [Member] | Debt Service Coverage Ratio Range Three [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans Receivable, Gross, Commercial, Mortgage	17	201
Mortgage Loans By Credit Quality Indicator Range One [Member] | Debt Service Coverage Ratio Range One [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans Receivable, Gross, Commercial, Mortgage	$ 40,195	$ 37,073