Investment Securities Held-to-Maturity - Unrealized Market Loss of Securities (Detail) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Schedule of Held-to-maturity Securities [Line Items]
Less Than 12 Months, Fair Value	$ 19,919	$ 207,932
Less Than 12 Months, Unrealized Losses	20	2,076
12 Months or Longer, Fair Value	780,410	1,369,175
12 Months or Longer, Unrealized Losses	16,027	55,810
Total, Fair Value	800,329	1,577,107
Total, Unrealized Losses	16,047	57,886
U.S. Treasury [Member]
Schedule of Held-to-maturity Securities [Line Items]
Less Than 12 Months, Fair Value		9,958
Less Than 12 Months, Unrealized Losses		2
Total, Fair Value		9,958
Total, Unrealized Losses		2
U.S. Government Sponsored Enterprises [Member]
Schedule of Held-to-maturity Securities [Line Items]
Less Than 12 Months, Fair Value		9,849
Less Than 12 Months, Unrealized Losses		42
12 Months or Longer, Fair Value	29,914	69,499
12 Months or Longer, Unrealized Losses	72	761
Total, Fair Value	29,914	79,348
Total, Unrealized Losses	72	803
SBA Backed Securities [Member]
Schedule of Held-to-maturity Securities [Line Items]
12 Months or Longer, Fair Value	25,057	49,987
12 Months or Longer, Unrealized Losses	224	2,065
Total, Fair Value	25,057	49,987
Total, Unrealized Losses	224	2,065
U.S. Government Agency and Sponsored Enterprises Mortgage-Backed Securities [Member]
Schedule of Held-to-maturity Securities [Line Items]
Less Than 12 Months, Fair Value	19,919	188,125
Less Than 12 Months, Unrealized Losses	20	2,032
12 Months or Longer, Fair Value	725,439	1,249,689
12 Months or Longer, Unrealized Losses	15,731	52,984
Total, Fair Value	745,358	1,437,814
Total, Unrealized Losses	$ 15,751	$ 55,016