Investment in Securities - Maturities of Investments (Detail) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Investments, Debt and Equity Securities [Abstract]
Bonds held-to-maturity securities, amortized cost, due in one year or less	$ 905,262
Bonds held-to-maturity securities, amortized cost, due after one year through five years	3,178,976
Bonds held-to-maturity securities, amortized cost, due after five years through ten years	3,403,404
Bonds held-to-maturity securities, amortized cost, due after ten years	1,088,550
Bonds held-to-maturity securities, amortized cost	8,576,192	$ 8,631,261
Bonds held-to-maturity securities, fair value, due in one year or less	907,767
Bonds held-to-maturity securities, fair value, due after one year through five years	3,191,015
Bonds held-to-maturity securities, fair value, due after five years through ten years	3,342,208
Bonds held-to-maturity securities, fair value, due after ten years	1,060,666
Bonds held-to-maturity securities, fair value	8,501,656	8,968,690
Bonds available-for-sale securities, amortized cost, due in one year or less	524,449
Bonds available-for-sale securities, amortized cost, due after one year through five years	3,040,598
Bonds available-for-sale securities, amortized cost, due after five years through ten years	2,180,525
Bonds available-for-sale securities, amortized cost, due after ten years	702,005
Bonds available-for-sale securities, amortized cost	6,447,577	6,435,670
Bonds available-for-sale securities, fair value, due in one year or less	524,574
Bonds available-for-sale securities, fair value, due after one year through five years	3,015,422
Bonds available-for-sale securities, fair value, due after five years through ten years	2,207,898
Bonds available-for-sale securities, fair value, due after ten years	732,480
Bonds available-for-sale securities, fair value	$ 6,480,374	$ 6,725,085