Investment Securities - Securities with Unrealized Losses Not Recognized in Income (Detail) - USD ($) $ in Thousands	Mar. 31, 2016	Dec. 31, 2015
Estimated Fair Value
Estimated Fair Value, Less than 12 Months, Available-for-sale	$ 79,766	$ 387,089
Estimated Fair Value, 12 Months or Longer, Available-for-sale	30,090	31,273
Total, Estimated Fair Value, Available-for-sale	109,856	418,362
Unrealized Losses
Unrealized Losses, Less than 12 Months, Available-for-sale	3,246	3,950
Unrealized Losses,12 Months or Longer, Available-for-sale	96	692
Total, Unrealized Losses, Available-for-sale	3,342	4,642
Estimated Fair Value
Estimated Fair Value, Less than 12 Months, Held-to-maturity	68,424	188,026
Estimated Fair Value, 12 Months or Longer, Held-to-maturity	140,452	156,182
Total, Estimated Fair Value, Held-to-maturity	208,876	344,208
Held-to-maturity Securities, Continuous Unrealized Loss Position, Accumulated Loss	3,774	5,741
Unrealized Losses
Unrealized Losses, Less than 12 Months, Held-to-maturity	2,699	2,092
Unrealized Losses, 12 Months or Longer, Held-to-maturity	1,075	3,649
Corporate bonds
Estimated Fair Value
Estimated Fair Value, Less than 12 Months, Available-for-sale	35,791	8,237
Estimated Fair Value, 12 Months or Longer, Available-for-sale	0	0
Total, Estimated Fair Value, Available-for-sale	35,791	8,237
Unrealized Losses
Unrealized Losses, Less than 12 Months, Available-for-sale	3,170	227
Unrealized Losses,12 Months or Longer, Available-for-sale	0	0
Total, Unrealized Losses, Available-for-sale	3,170	227
Mortgage-backed securities—residential issued by government sponsored entities
Estimated Fair Value
Estimated Fair Value, Less than 12 Months, Available-for-sale	43,975	378,852
Estimated Fair Value, 12 Months or Longer, Available-for-sale	30,090	31,273
Total, Estimated Fair Value, Available-for-sale	74,065	410,125
Unrealized Losses
Unrealized Losses, Less than 12 Months, Available-for-sale	76	3,723
Unrealized Losses,12 Months or Longer, Available-for-sale	96	692
Total, Unrealized Losses, Available-for-sale	172	4,415
Corporate bonds
Estimated Fair Value
Estimated Fair Value, Less than 12 Months, Held-to-maturity	41,391	0
Estimated Fair Value, 12 Months or Longer, Held-to-maturity	4,625	13,035
Total, Estimated Fair Value, Held-to-maturity	46,016	13,035
Held-to-maturity Securities, Continuous Unrealized Loss Position, Accumulated Loss	3,000	351
Unrealized Losses
Unrealized Losses, Less than 12 Months, Held-to-maturity	2,571	0
Unrealized Losses, 12 Months or Longer, Held-to-maturity	429	351
U.S. Government agencies
Estimated Fair Value
Estimated Fair Value, Less than 12 Months, Held-to-maturity	0	44,658
Estimated Fair Value, 12 Months or Longer, Held-to-maturity	12,947	0
Total, Estimated Fair Value, Held-to-maturity	12,947	44,658
Held-to-maturity Securities, Continuous Unrealized Loss Position, Accumulated Loss	100	401
Unrealized Losses
Unrealized Losses, Less than 12 Months, Held-to-maturity	0	401
Unrealized Losses, 12 Months or Longer, Held-to-maturity	100	0
Mortgage-backed securities—residential issued by government sponsored entities
Estimated Fair Value
Estimated Fair Value, Less than 12 Months, Held-to-maturity	27,033	143,368
Estimated Fair Value, 12 Months or Longer, Held-to-maturity	122,880	143,147
Total, Estimated Fair Value, Held-to-maturity	149,913	286,515
Held-to-maturity Securities, Continuous Unrealized Loss Position, Accumulated Loss	674	4,989
Unrealized Losses
Unrealized Losses, Less than 12 Months, Held-to-maturity	128	1,691
Unrealized Losses, 12 Months or Longer, Held-to-maturity	$ 546	$ 3,298