Securities - Schedule of Amortized Cost and Fair Value of Available-for-Sale and Held-to-Maturity Securities by Contractual Maturity (Detail) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Amortized Cost And Fair Value Debt Securities [Abstract]
Total available-for-sale debt securities, Amortized Cost, Less than one year	$ 47,465
Total available-for-sale debt securities, Amortized Cost, 1-5 years	145,739
Total available-for-sale debt securities, Amortized Cost, 5-10 years	358,854
Total available-for-sale debt securities, Amortized Cost, Over 10 years	2,190,424
Available-for-sale, Amortized Cost	2,742,482	$ 1,916,658
Total held-to-maturity debt securities, Amortized Cost, Less than one year	11,759
Total held-to-maturity debt securities, Amortized Cost, 1-5 years	133,053
Total held-to-maturity debt securities, Amortized Cost, 5-10 years	228,148
Total held-to-maturity debt securities, Amortized Cost, Over 10 years	440,780
Held-to-maturity, Amortized Cost	813,740	731,212
Total debt securities, Amortized Cost	3,556,222
Total available-for-sale debt securities, Fair Value, Less than one year	47,583
Total available-for-sale debt securities, Fair Value, 1-5 years	152,800
Total available-for-sale debt securities, Fair Value, 5-10 years	366,119
Total available-for-sale debt securities, Fair Value, Over 10 years	2,208,710
Total available-for-sale debt securities, Fair Value	2,775,212	1,978,136
Total held-to-maturity debt securities, Fair Value, Less than one year	11,872
Total held-to-maturity debt securities, Fair Value, 1-5 years	140,315
Total held-to-maturity debt securities, Fair Value, 5-10 years	237,907
Total held-to-maturity debt securities, Fair Value, Over 10 years	449,778
Total held-to-maturity debt securities, Fair Value	839,872	768,183
Total debt securities, Fair Value	$ 3,615,084	$ 2,746,319