Investment Securities Held-to-Maturity - Unrealized Market Loss of Securities (Detail) - USD ($) $ in Thousands	Jun. 30, 2017	Dec. 31, 2016
Schedule of Held-to-maturity Securities [Line Items]
Less Than 12 Months, Fair Value	$ 1,111,732	$ 1,113,231
Less Than 12 Months, Unrealized Losses	20,030	22,340
12 Months or Longer, Fair Value	76,584	58,535
12 Months or Longer, Unrealized Losses	2,358	1,852
Total, Fair Value	1,188,316	1,171,766
Total, Unrealized Losses	22,388	24,192
U.S. Government Sponsored Enterprises [Member]
Schedule of Held-to-maturity Securities [Line Items]
Less Than 12 Months, Fair Value	45,209	59,219
Less Than 12 Months, Unrealized Losses	287	527
Total, Fair Value	45,209	59,219
Total, Unrealized Losses	287	527
SBA Backed Securities [Member]
Schedule of Held-to-maturity Securities [Line Items]
Less Than 12 Months, Fair Value	50,633	45,052
Less Than 12 Months, Unrealized Losses	779	1,088
Total, Fair Value	50,633	45,052
Total, Unrealized Losses	779	1,088
U.S. Government Agency and Sponsored Mortgage-Backed Securities [Member]
Schedule of Held-to-maturity Securities [Line Items]
Less Than 12 Months, Fair Value	1,015,890	1,008,960
Less Than 12 Months, Unrealized Losses	18,964	20,725
12 Months or Longer, Fair Value	76,584	58,535
12 Months or Longer, Unrealized Losses	2,358	1,852
Total, Fair Value	1,092,474	1,067,495
Total, Unrealized Losses	$ 21,322	$ 22,577