Investment in Securities - Maturities of Investments (Detail) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Investments, Debt and Equity Securities [Abstract]
Bonds held-to-maturity securities, amortized cost, due in one year or less	$ 497,000
Bonds held-to-maturity securities, amortized cost, due after one year through five years	4,023,951
Bonds held-to-maturity securities, amortized cost, due after five years through ten years	2,978,311
Bonds held-to-maturity securities, amortized cost, due after ten years	708,867
Bonds held-to-maturity securities, amortized cost	8,208,129	$ 8,211,449
Bonds held-to-maturity securities, fair value, due in one year or less	501,658
Bonds held-to-maturity securities, fair value, due after one year through five years	4,105,063
Bonds held-to-maturity securities, fair value, due after five years through ten years	3,004,559
Bonds held-to-maturity securities, fair value, due after ten years	712,320
Bonds held-to-maturity securities, fair value	8,323,600	8,130,084
Bonds available-for-sale securities, amortized cost, due in one year or less	138,599
Bonds available-for-sale securities, amortized cost, due after one year through five years	3,169,190
Bonds available-for-sale securities, amortized cost, due after five years through ten years	2,640,729
Bonds available-for-sale securities, amortized cost, due after ten years	527,766
Bonds available-for-sale securities, amortized cost	6,476,284	6,261,621
Bonds available-for-sale securities, fair value, due in one year or less	140,060
Bonds available-for-sale securities, fair value, due after one year through five years	3,227,783
Bonds available-for-sale securities, fair value, due after five years through ten years	2,680,370
Bonds available-for-sale securities, fair value, due after ten years	536,180
Bonds available-for-sale securities, fair value	$ 6,584,393	$ 6,215,563