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Fair Value Measurements - Additional Information (Details) - USD ($)
3 Months Ended
Mar. 31, 2018
Dec. 31, 2017
Mar. 31, 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Fair Value Inputs, Comparability Adjustments 0.00%    
Fair Value, Assets, Level 1 to Level 2 Transfers, Amount $ 283,000,000   $ 230,000,000
Fair Value, Assets, Level 2 to Level 1 Transfers, 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%  
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% 21.00%