Investment Securities - Schedule of Amortized Cost and Estimated Fair Value of Investment Securities by Maturity (Detail) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Investments, Debt and Equity Securities [Abstract]
Securities Available for Sale, Weighted Average Yield, Within one year or less	2.16%
Securities Available for Sale, Weighted Average Yield, One through five years	1.73%
Securities Available for Sale, Weighted Average Yield, After five through ten years	2.17%
Securities Available for Sale, Weighted Average Yield, Over ten years	2.18%
Securities Available for Sale, Weighted Average Yield, Total	2.12%
Securities Available for Sale, Amortized Cost, Within one year or less	$ 84,514
Securities Available for Sale, Amortized Cost, One through five years	279,674
Securities Available for Sale, Amortized Cost, After five through ten years	406,800
Securities Available for Sale, Amortized Cost, Over ten years	1,545,702
Securities Available for Sale, Amortized Cost, Total	2,316,690
Securities Available for Sale, Estimated Fair Value, Within one year or less	84,638
Securities Available for Sale, Estimated Fair Value, One through five years	282,952
Securities Available for Sale, Estimated Fair Value, After five through ten years	414,109
Securities Available for Sale, Estimated Fair Value, Over ten years	1,560,914
Securities Available for Sale, Estimated Fair Value, Totals	2,342,613	2,158,853
Securities Held to Maturity, Weighted Average Yield, Within one year or less	2.65%
Securities Held to Maturity, Weighted Average Yield, One through five years	2.80%
Securities Held to Maturity, Weighted Average Yield, After five through ten years	3.01%
Securities Held to Maturity, Weighted Average Yield, Over ten years	3.03%
Securities Held to Maturity, Weighted Average Yield, Totals	2.97%
Securities Held to Maturity, Amortized Cost, Within one year or less	10,075
Securities Held to Maturity, Amortized Cost, One through five years	12,933
Securities Held to Maturity, Amortized Cost, After five through ten years	20,128
Securities Held to Maturity, Amortized Cost, Over ten years	70,306
Securities held to maturity, Amortized Cost	113,442	116,960
Securities Held to Maturity, Estimated Fair Value, Within one year or less	10,103
Securities Held to Maturity, Estimated Fair Value, One through five years	13,256
Securities Held to Maturity, Estimated Fair Value, After five through ten years	21,005
Securities Held to Maturity, Estimated Fair Value, Over ten years	72,007
Securities Held to Maturity, Estimated Fair Value, Totals	$ 116,371	$ 119,481