Investments (Credit Quality of Commercial Mortgage Loans) (Details) - USD ($) $ in Millions	Jun. 30, 2019	Dec. 31, 2018
Accounts, Notes, Loans and Financing Receivable [Line Items]
Mortgage Loans, Gross	$ 78,092	$ 75,795
Loans Receivable Commercial Mortgage Percentage	100.00%	100.00%
Estimated Fair Value Commercial Mortgage Percentage	100.00%	100.00%
Less than 65%
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans Receivable Commercial Mortgage Percentage	85.40%	85.20%
Estimated Fair Value Commercial Mortgage Percentage	85.70%	85.30%
65% to 75%
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans Receivable Commercial Mortgage Percentage	11.80%	12.00%
Estimated Fair Value Commercial Mortgage Percentage	11.70%	12.00%
76% to 80%
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans Receivable Commercial Mortgage Percentage	1.30%	1.40%
Estimated Fair Value Commercial Mortgage Percentage	1.30%	1.40%
Greater than 80%
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans Receivable Commercial Mortgage Percentage	1.50%	1.40%
Estimated Fair Value Commercial Mortgage Percentage	1.30%	1.30%
Commercial Mortgage Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Mortgage Loans, Gross	$ 49,570	$ 48,463
Fair Value Mortgage Loans Net Not On Recurring Basis	51,232	48,747
Commercial Mortgage Loans | Greater than 1.20x
Accounts, Notes, Loans and Financing Receivable [Line Items]
Mortgage Loans, Gross	47,932	47,069
Commercial Mortgage Loans | 1.00x - 1.20x
Accounts, Notes, Loans and Financing Receivable [Line Items]
Mortgage Loans, Gross	1,261	1,212
Commercial Mortgage Loans | Less than 1.00x
Accounts, Notes, Loans and Financing Receivable [Line Items]
Mortgage Loans, Gross	377	182
Commercial Mortgage Loans | Less than 65%
Accounts, Notes, Loans and Financing Receivable [Line Items]
Mortgage Loans, Gross	42,353	41,288
Fair Value Mortgage Loans Net Not On Recurring Basis	43,922	41,599
Commercial Mortgage Loans | Less than 65% | Greater than 1.20x
Accounts, Notes, Loans and Financing Receivable [Line Items]
Mortgage Loans, Gross	41,424	40,360
Commercial Mortgage Loans | Less than 65% | 1.00x - 1.20x
Accounts, Notes, Loans and Financing Receivable [Line Items]
Mortgage Loans, Gross	773	827
Commercial Mortgage Loans | Less than 65% | Less than 1.00x
Accounts, Notes, Loans and Financing Receivable [Line Items]
Mortgage Loans, Gross	156	101
Commercial Mortgage Loans | 65% to 75%
Accounts, Notes, Loans and Financing Receivable [Line Items]
Mortgage Loans, Gross	5,841	5,815
Fair Value Mortgage Loans Net Not On Recurring Basis	5,975	5,849
Commercial Mortgage Loans | 65% to 75% | Greater than 1.20x
Accounts, Notes, Loans and Financing Receivable [Line Items]
Mortgage Loans, Gross	5,649	5,790
Commercial Mortgage Loans | 65% to 75% | 1.00x - 1.20x
Accounts, Notes, Loans and Financing Receivable [Line Items]
Mortgage Loans, Gross	26	0
Commercial Mortgage Loans | 65% to 75% | Less than 1.00x
Accounts, Notes, Loans and Financing Receivable [Line Items]
Mortgage Loans, Gross	166	25
Commercial Mortgage Loans | 76% to 80%
Accounts, Notes, Loans and Financing Receivable [Line Items]
Mortgage Loans, Gross	660	688
Fair Value Mortgage Loans Net Not On Recurring Basis	645	664
Commercial Mortgage Loans | 76% to 80% | Greater than 1.20x
Accounts, Notes, Loans and Financing Receivable [Line Items]
Mortgage Loans, Gross	377	423
Commercial Mortgage Loans | 76% to 80% | 1.00x - 1.20x
Accounts, Notes, Loans and Financing Receivable [Line Items]
Mortgage Loans, Gross	228	209
Commercial Mortgage Loans | 76% to 80% | Less than 1.00x
Accounts, Notes, Loans and Financing Receivable [Line Items]
Mortgage Loans, Gross	55	56
Commercial Mortgage Loans | Greater than 80%
Accounts, Notes, Loans and Financing Receivable [Line Items]
Mortgage Loans, Gross	716	672
Fair Value Mortgage Loans Net Not On Recurring Basis	690	635
Commercial Mortgage Loans | Greater than 80% | Greater than 1.20x
Accounts, Notes, Loans and Financing Receivable [Line Items]
Mortgage Loans, Gross	482	496
Commercial Mortgage Loans | Greater than 80% | 1.00x - 1.20x
Accounts, Notes, Loans and Financing Receivable [Line Items]
Mortgage Loans, Gross	234	176
Commercial Mortgage Loans | Greater than 80% | Less than 1.00x
Accounts, Notes, Loans and Financing Receivable [Line Items]
Mortgage Loans, Gross	$ 0	$ 0