Investment Securities - Expected Maturities of Investment Securities Portfolio (Detail) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Investments Debt And Equity Securities [Abstract]
Available-for-Sale, Maturity, Within one year, Amortized Cost	$ 356,218
Available-for-Sale, Maturity, One to five years, Amortized Cost	2,714,931
Available-for-Sale, Maturity, Five to ten years, Amortized Cost	1,016,686
Available-for-Sale, Maturity, Beyond ten years, Amortized Cost	1,225,374
Available-for-Sale, Amortized Cost	5,313,209	$ 4,172,617
Available-for-Sale, Maturity, Within one year, Fair Value	357,345
Available-for-Sale, Maturity, One to five years, Fair Value	2,747,339
Available-for-Sale, Maturity, Five to ten years, Fair Value	1,033,597
Available-for-Sale, Maturity, Beyond ten years, Fair Value	1,246,810
Available-for-Sale securities, Fair Value	$ 5,385,091	$ 4,123,416
Available-for-Sale, Maturity, Within one year, Weighted Average Yield	2.28%
Available-for-Sale, Maturity, One to five years, Weighted Average Yield	2.81%
Available-for-Sale, Maturity, Five to ten years, Weighted Average Yield	3.11%
Available-for-Sale, Maturity, Beyond ten years, Weighted Average Yield	3.35%
Available-for-Sale, Weighted Average Yield	2.96%