Fair Value Measurements - Financial Instruments Not Carried At Fair Value (Details) - USD ($) $ in Millions	Sep. 30, 2018	Jun. 01, 2018	Dec. 31, 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets	$ 159,103	$ 162,284	$ 168,732
Assets, Fair Value Disclosure	119,130		137,619
Liabilities	157,334	$ 160,517	162,052
Fair Value, Inputs, Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets, Fair Value Disclosure	1,207		1,606
Reported Value Measurement [Member] | Fair Value, Inputs, Level 3 [Member] | Other Liabilities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Liabilities	0		8
Reported Value Measurement [Member] | Fair Value, Inputs, Level 3 [Member] | Other Policyholder Funds and Benefits Payable [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Liabilities	6,277		5,905
Reported Value Measurement [Member] | Fair Value, Inputs, Level 3 [Member] | Investment Contracts [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Liabilities	355		342
Reported Value Measurement [Member] | Fair Value, Inputs, Level 3 [Member] | Policy Loans [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets	1,426		1,432
Reported Value Measurement [Member] | Fair Value, Inputs, Level 3 [Member] | Mortgages [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets	2,099		2,872
Estimate of Fair Value Measurement [Member] | Fair Value, Inputs, Level 3 [Member] | Other Liabilities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial Liabilities Fair Value Disclosure	0		8
Estimate of Fair Value Measurement [Member] | Fair Value, Inputs, Level 3 [Member] | Other Policyholder Funds and Benefits Payable [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial Liabilities Fair Value Disclosure	5,971		6,095
Estimate of Fair Value Measurement [Member] | Fair Value, Inputs, Level 3 [Member] | Investment Contracts [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial Liabilities Fair Value Disclosure	355		361
Estimate of Fair Value Measurement [Member] | Fair Value, Inputs, Level 3 [Member] | Policy Loans [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets, Fair Value Disclosure	1,426		1,432
Estimate of Fair Value Measurement [Member] | Fair Value, Inputs, Level 3 [Member] | Mortgages [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets, Fair Value Disclosure	$ 2,081		$ 2,941