Investment Securities Held-to-Maturity - Unrealized Market Loss of Securities (Detail) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Schedule of Held-to-maturity Securities [Line Items]
Less Than 12 Months, Fair Value	$ 59,831	$ 207,932
Less Than 12 Months, Unrealized Losses	439	2,076
12 Months or Longer, Fair Value	1,361,426	1,369,175
12 Months or Longer, Unrealized Losses	41,432	55,810
Total, Fair Value	1,421,257	1,577,107
Total, Unrealized Losses	41,871	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	74,876	69,499
12 Months or Longer, Unrealized Losses	417	761
Total, Fair Value	74,876	79,348
Total, Unrealized Losses	417	803
SBA Backed Securities [Member]
Schedule of Held-to-maturity Securities [Line Items]
12 Months or Longer, Fair Value	49,502	49,987
12 Months or Longer, Unrealized Losses	1,295	2,065
Total, Fair Value	49,502	49,987
Total, Unrealized Losses	1,295	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	59,831	188,125
Less Than 12 Months, Unrealized Losses	439	2,032
12 Months or Longer, Fair Value	1,237,048	1,249,689
12 Months or Longer, Unrealized Losses	39,720	52,984
Total, Fair Value	1,296,879	1,437,814
Total, Unrealized Losses	$ 40,159	$ 55,016