Commitments and Contingencies - Fair Value of Annuities (Detail) $ in Thousands	Dec. 31, 2016 USD ($)
Commitments And Contingencies [Line Items]
Statement value of annuities exceeding 1% of statutory surplus	$ 96,056
Metropolitan Life Insurance Company (A+) [Member]
Commitments And Contingencies [Line Items]
Statement value of annuities exceeding 1% of statutory surplus	17,098
Pacific Life and Annuity Company (A+) [Member]
Commitments And Contingencies [Line Items]
Statement value of annuities exceeding 1% of statutory surplus	16,688
American General Life Insurance (A) [Member]
Commitments And Contingencies [Line Items]
Statement value of annuities exceeding 1% of statutory surplus	15,049
New York Life Insurance Company (A++) [Member]
Commitments And Contingencies [Line Items]
Statement value of annuities exceeding 1% of statutory surplus	11,408
John Hancock Life Insurance Company USA (A+) [Member]
Commitments And Contingencies [Line Items]
Statement value of annuities exceeding 1% of statutory surplus	6,724
Athene Annuity and Life Assurance Company (A-) [Member]
Commitments And Contingencies [Line Items]
Statement value of annuities exceeding 1% of statutory surplus	5,164
Other [Member]
Commitments And Contingencies [Line Items]
Statement value of annuities exceeding 1% of statutory surplus	$ 23,925