Securities - Amortized Cost and Estimated Fair Value of Available-for-Sale Securities (Detail) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020	Sep. 30, 2020
Investments, Debt and Equity Securities [Abstract]
Available-for-Sale, Amortized Cost Basis, Due within one year	$ 473,997
Available-for-Sale, Amortized Cost Basis, Due after one year through five years	2,934,396
Available-for-Sale, Amortized Cost Basis, Due after five years through ten years	2,485,328
Available-for-Sale, Amortized Cost Basis, Due after ten years	87,653
Available-for-Sale, Amortized Cost Basis, Total	5,981,374	$ 4,177,179	$ 4,238,624
Available-for-Sale, Estimated Fair Value, Due within one year	478,547
Available-for-Sale, Estimated Fair Value, Due after one year through five years	3,023,175
Available-for-Sale, Estimated Fair Value, Due after five years through ten years	2,531,243
Available-for-Sale, Estimated Fair Value, Due after ten years	87,019
Available-for-sale investment securities, Total Fair Value	$ 6,119,984	$ 4,393,029	$ 4,431,280