Investment in Securities - Maturities of Investments (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Investments, Debt and Equity Securities [Abstract]
Bonds Held-to-Maturity, Amortized Cost, Due in one year or less	$ 383,192
Bonds Held-to-Maturity, Amortized Cost, Due after one year through five years	4,214,944
Bonds Held-to-Maturity, Amortized Cost, Due after five years through ten years	2,968,600
Bonds Held-to-Maturity, Amortized Cost, Due after ten years	644,713
Held-to-maturity securities, cost or amortized cost	8,211,449	$ 7,552,959
Bonds Held-to-Maturity, Fair Value, Due in one year or less	386,893
Bonds Held-to-Maturity, Fair Value, Due after one year through five years	4,228,048
Bonds Held-to-Maturity, Fair Value, Due after five years through ten years	2,888,783
Bonds Held-to-Maturity, Fair Value, Due after ten years	626,360
Held-to-maturity securities, fair value	8,130,084	7,774,353
Bonds Available-for-Sale, Amortized Cost, Due in one year or less	76,516
Bonds Available-for-Sale, Amortized Cost, Due after one year through five years	2,841,372
Bonds Available-for-Sale, Amortized Cost, Due after five years through ten years	2,846,375
Bonds Available-for-Sale, Amortized Cost, Due after ten years	497,358
Available-for-sale debt securities, cost or amortized cost	6,261,621	5,957,901
Bonds Available-for-Sale, Fair Value, Due in one year or less	77,149
Bonds Available-for-Sale, Fair Value, Due after one year through five years	2,852,085
Bonds Available-for-Sale, Fair Value, Due after five years through ten years	2,791,623
Bonds Available-for-Sale, Fair Value, Due after ten years	494,706
Available-for-sale debt securities, fair value	$ 6,215,563	$ 6,145,308