Revenue - Additional Information (Detail) $ in Millions	3 Months Ended
Revenue - Additional Information (Detail) $ in Millions	Mar. 31, 2018 USD ($)
Schedule of Investment Advisory Administration Fees and Securities Lending Revenue and Performance Fees by Type [line Items]
Estimated annual fixed minimum fees for currently outstanding contracts	$ 348
Minimum [Member]
Schedule of Investment Advisory Administration Fees and Securities Lending Revenue and Performance Fees by Type [line Items]
Term of currently outstanding contracts	1 year
Maximum [Member]
Schedule of Investment Advisory Administration Fees and Securities Lending Revenue and Performance Fees by Type [line Items]
Term of currently outstanding contracts	5 years