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, 2020	Dec. 31, 2019
Amortized Cost And Fair Value Debt Securities [Abstract]
Total available-for-sale debt securities, Amortized Cost, Less than one year	$ 46,569
Total available-for-sale debt securities, Amortized Cost, 1-5 years	137,986
Total available-for-sale debt securities, Amortized Cost, 5-10 years	323,482
Total available-for-sale debt securities, Amortized Cost, Over 10 years	1,677,928
Available-for-sale, Amortized Cost	2,185,965	$ 2,369,416
Total held-to-maturity debt securities, Amortized Cost, Less than one year	18,699
Total held-to-maturity debt securities, Amortized Cost, 1-5 years	122,987
Total held-to-maturity debt securities, Amortized Cost, 5-10 years	300,223
Total held-to-maturity debt securities, Amortized Cost, Over 10 years	372,505
Held-to-maturity, Amortized Cost	814,414	851,753
Total debt securities, Amortized Cost	3,000,379
Total available-for-sale debt securities, Fair Value, Less than one year	46,618
Total available-for-sale debt securities, Fair Value, 1-5 years	142,442
Total available-for-sale debt securities, Fair Value, 5-10 years	338,039
Total available-for-sale debt securities, Fair Value, Over 10 years	1,734,983
Total available-for-sale debt securities, Fair Value	2,262,082	2,393,558
Total held-to-maturity debt securities, Fair Value, Less than one year	18,756
Total held-to-maturity debt securities, Fair Value, 1-5 years	127,113
Total held-to-maturity debt securities, Fair Value, 5-10 years	308,896
Total held-to-maturity debt securities, Fair Value, Over 10 years	386,355
Total held-to-maturity debt securities, Fair Value	841,120	874,523
Total debt securities, Fair Value	$ 3,103,202	$ 3,268,081