Investment in Securities - Maturities of Investments (Detail) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Investments, Debt and Equity Securities [Abstract]
Bonds held-to-maturity securities, amortized cost, due in one year or less	$ 430,988
Bonds held-to-maturity securities, amortized cost, due after one year through five years	4,033,584
Bonds held-to-maturity securities, amortized cost, due after five years through ten years	3,099,228
Bonds held-to-maturity securities, amortized cost, due after ten years	830,357
Bonds held-to-maturity securities, amortized cost	8,394,157	$ 8,211,449
Bonds held-to-maturity securities, fair value, due in one year or less	436,069
Bonds held-to-maturity securities, fair value, due after one year through five years	4,153,917
Bonds held-to-maturity securities, fair value, due after five years through ten years	3,223,700
Bonds held-to-maturity securities, fair value, due after ten years	854,913
Bonds held-to-maturity securities, fair value	8,668,599	8,130,084
Bonds available-for-sale securities, amortized cost, due in one year or less	243,111
Bonds available-for-sale securities, amortized cost, due after one year through five years	3,211,961
Bonds available-for-sale securities, amortized cost, due after five years through ten years	2,513,056
Bonds available-for-sale securities, amortized cost, due after ten years	641,229
Bonds available-for-sale securities, amortized cost	6,609,357	6,261,621
Bonds available-for-sale securities, fair value, due in one year or less	245,596
Bonds available-for-sale securities, fair value, due after one year through five years	3,313,108
Bonds available-for-sale securities, fair value, due after five years through ten years	2,623,952
Bonds available-for-sale securities, fair value, due after ten years	660,261
Bonds available-for-sale securities, fair value	$ 6,842,917	$ 6,215,563