Securities - Schedule of Amortized Cost and Fair Value of Available-for-Sale and Held-to-Maturity Securities by Contractual Maturity (Detail) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Amortized Cost And Fair Value Debt Securities [Abstract]
Total available-for-sale debt securities, Amortized Cost, Less than one year	$ 52,867
Total available-for-sale debt securities, Amortized Cost, 1-5 years	159,663
Total available-for-sale debt securities, Amortized Cost, 5-10 years	367,366
Total available-for-sale debt securities, Amortized Cost, Over 10 years	1,336,762
Available-for-sale, Amortized Cost	1,916,658	$ 2,369,416
Total held-to-maturity debt securities, Amortized Cost, Less than one year	8,051
Total held-to-maturity debt securities, Amortized Cost, 1-5 years	116,033
Total held-to-maturity debt securities, Amortized Cost, 5-10 years	235,993
Total held-to-maturity debt securities, Amortized Cost, Over 10 years	371,135
Held-to-maturity, Amortized Cost	731,212	851,753
Total debt securities, Amortized Cost	2,647,870
Total available-for-sale debt securities, Fair Value, Less than one year	52,943
Total available-for-sale debt securities, Fair Value, 1-5 years	167,823
Total available-for-sale debt securities, Fair Value, 5-10 years	380,117
Total available-for-sale debt securities, Fair Value, Over 10 years	1,377,253
Total available-for-sale debt securities, Fair Value	1,978,136	2,393,558
Total held-to-maturity debt securities, Fair Value, Less than one year	8,110
Total held-to-maturity debt securities, Fair Value, 1-5 years	122,589
Total held-to-maturity debt securities, Fair Value, 5-10 years	248,079
Total held-to-maturity debt securities, Fair Value, Over 10 years	389,405
Total held-to-maturity debt securities, Fair Value	768,183	874,523
Total debt securities, Fair Value	$ 2,746,319	$ 3,268,081