Investment in Securities - Maturities of Investments (Detail) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Investments, Debt and Equity Securities [Abstract]
Bonds held-to-maturity securities, amortized cost, due in one year or less	$ 541,108
Bonds held-to-maturity securities, amortized cost, due after one year through five years	3,890,976
Bonds held-to-maturity securities, amortized cost, due after five years through ten years	3,373,279
Bonds held-to-maturity securities, amortized cost, due after ten years	910,206
Bonds held-to-maturity securities, amortized cost	8,715,569	$ 8,211,449
Bonds held-to-maturity securities, fair value, due in one year or less	547,409
Bonds held-to-maturity securities, fair value, due after one year through five years	4,018,966
Bonds held-to-maturity securities, fair value, due after five years through ten years	3,546,108
Bonds held-to-maturity securities, fair value, due after ten years	951,205
Bonds held-to-maturity securities, fair value	9,063,688	8,130,084
Bonds available-for-sale securities, amortized cost, due in one year or less	437,371
Bonds available-for-sale securities, amortized cost, due after one year through five years	3,076,222
Bonds available-for-sale securities, amortized cost, due after five years through ten years	2,440,494
Bonds available-for-sale securities, amortized cost, due after ten years	591,271
Bonds available-for-sale securities, amortized cost	6,545,358	6,261,621
Bonds available-for-sale securities, fair value, due in one year or less	440,611
Bonds available-for-sale securities, fair value, due after one year through five years	3,188,171
Bonds available-for-sale securities, fair value, due after five years through ten years	2,580,319
Bonds available-for-sale securities, fair value, due after ten years	617,144
Bonds available-for-sale securities, fair value	$ 6,826,245	$ 6,215,563