Level 4~Derivatives - Additional Information (Details) - USD ($)	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Notional amount	$ 12,875,000,000		$ 12,730,000,000
Deferred net gains on derivative instruments recorded in AOCI that are expected to be reclassified to earnings during the next twelve months	$ 28,000,000
Maximum term over which company is hedging its exposure	1 year
Collateral receivable	$ 1,000,000		1,000,000
Pledged collateral, securities	93,000,000		101,000,000
Margin Deposit Assets	92		96
Cash collateral held	14,000,000		11,000,000
Securities Received as Collateral	2,000,000
Not Designated as Hedging Instrument
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Notional amount	10,546,000,000		10,387,000,000
Designated as Hedging Instrument | Cash Flow Hedging
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Ineffectiveness recognized in income with net realized capital gains (losses)	0	$ 0
Interest Rate Swap [Member] | Not Designated as Hedging Instrument
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Notional amount	7,200,000,000		7,300,000,000
Interest Rate Swap [Member] | Designated as Hedging Instrument
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Notional amount	2,176,000,000		$ 2,190,000,000
AOCI Attributable to Parent [Member]
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Loss on Discontinuation of Cash Flow Hedge Due to Forecasted Transaction Probable of Not Occurring	$ 0	$ 0