Commitments and Contingencies - Fair Value of Annuities (Detail) $ in Thousands	Dec. 31, 2017 USD ($)
Commitments And Contingencies [Line Items]
Statement value of annuities exceeding 1% of statutory surplus	$ 95,936
Pacific Life and Annuity Company [Member] | A.M. Best Rating, A+ [Member]
Commitments And Contingencies [Line Items]
Statement value of annuities exceeding 1% of statutory surplus	17,068
American General Life Insurance Company [Member] | A.M. Best Rating, A [Member]
Commitments And Contingencies [Line Items]
Statement value of annuities exceeding 1% of statutory surplus	14,952
New York Life Insurance Company [Member] | A.M. Best Rating, A++ [Member]
Commitments And Contingencies [Line Items]
Statement value of annuities exceeding 1% of statutory surplus	11,312
Travelers Life and Annuity [Member] | A.M. Best Rating, A [Member]
Commitments And Contingencies [Line Items]
Statement value of annuities exceeding 1% of statutory surplus	10,471
Metropolitan Life Insurance Company [Member] | A.M. Best Rating, A+ [Member]
Commitments And Contingencies [Line Items]
Statement value of annuities exceeding 1% of statutory surplus	6,993
John Hancock Life Insurance Company [Member] | A.M. Best Rating, A+ [Member]
Commitments And Contingencies [Line Items]
Statement value of annuities exceeding 1% of statutory surplus	6,284
Athene Annuity and Life Company [Member] | A.M. Best Rating, A [Member]
Commitments And Contingencies [Line Items]
Statement value of annuities exceeding 1% of statutory surplus	4,760
United of Omaha [Member] | A.M. Best Rating, A+ [Member]
Commitments And Contingencies [Line Items]
Statement value of annuities exceeding 1% of statutory surplus	4,375
Other [Member]
Commitments And Contingencies [Line Items]
Statement value of annuities exceeding 1% of statutory surplus	$ 19,721