Fair Value Measurements - Level 4 Additional Information (Details) - USD ($)	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value assets level 1 to level 2 Transfer amount	$ 321,000,000	$ 283,000,000
Fair value assets level 2 to level 1 transfer amount	$ 0	$ 0
Separate Accounts [Member] | Hedge Funds [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share, Redemption Restriction, Percentage	51.00%		51.00%
Separate Accounts [Member] | Private Equity Funds [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share, Redemption Restriction, Percentage	0.00%		0.00%