Investment in Securities - Maturities of Investments (Detail) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Investments, Debt and Equity Securities [Abstract]
Bonds held-to-maturity, amortized cost, due in one year or less	$ 781,746
Bonds held-to-maturity, amortized cost, due after one year through five years	3,446,425
Bonds held-to-maturity, amortized cost, due after five years through ten years	3,398,612
Bonds held-to-maturity, amortized cost, due after ten years	1,004,478
Held-to-maturity securities, cost or amortized cost	8,631,261	$ 8,211,449
Bonds held-to-maturity, fair value, due in one year or less	791,175
Bonds held-to-maturity, fair value, due after one year through five years	3,566,116
Bonds held-to-maturity, fair value, due after five years through ten years	3,572,333
Bonds held-to-maturity, fair value, due after ten years	1,039,066
Held-to-maturity securities, fair value	8,968,690	8,130,084
Bonds available-for-sale, amortized cost, due in one year or less	450,482
Bonds available-for-sale, amortized cost, due after one year through five years	3,063,725
Bonds available-for-sale, amortized cost, due after five years through ten years	2,326,706
Bonds available-for-sale, amortized cost, due after ten years	594,757
Available-for-sale debt securities, cost or amortized cost	6,435,670	6,261,621
Bonds available-for-sale, fair value, due in one year or less	454,992
Bonds available-for-sale, fair value, due after one year through five years	3,185,728
Bonds available-for-sale, fair value, due after five years through ten years	2,465,073
Bonds available-for-sale, fair value, due after ten years	619,292
Available-for-sale debt securities, fair value	$ 6,725,085	$ 6,215,563