Investment Securities Held-to-Maturity - Unrealized Market Loss of Securities (Detail) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Schedule of Held-to-maturity Securities [Line Items]
Less Than 12 Months, Fair Value	$ 187,538	$ 207,932
Less Than 12 Months, Unrealized Losses	904	2,076
12 Months or Longer, Fair Value	440,668	1,369,175
12 Months or Longer, Unrealized Losses	5,782	55,810
Total, Fair Value	628,206	1,577,107
Total, Unrealized Losses	6,686	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	9,976	9,849
Less Than 12 Months, Unrealized Losses	22	42
12 Months or Longer, Fair Value	9,954	69,499
12 Months or Longer, Unrealized Losses	45	761
Total, Fair Value	19,930	79,348
Total, Unrealized Losses	67	803
SBA Backed Securities [Member]
Schedule of Held-to-maturity Securities [Line Items]
Less Than 12 Months, Fair Value	13,673
Less Than 12 Months, Unrealized Losses	66
12 Months or Longer, Fair Value		49,987
12 Months or Longer, Unrealized Losses		2,065
Total, Fair Value	13,673	49,987
Total, Unrealized Losses	66	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	163,889	188,125
Less Than 12 Months, Unrealized Losses	816	2,032
12 Months or Longer, Fair Value	430,714	1,249,689
12 Months or Longer, Unrealized Losses	5,737	52,984
Total, Fair Value	594,603	1,437,814
Total, Unrealized Losses	$ 6,553	$ 55,016