Investment Securities - Amortized Cost and Estimated Fair Value of Debt Securities by Contractual Maturity (Detail) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2014	Dec. 31, 2013	Mar. 31, 2013
Investments, Debt and Equity Securities [Abstract]
Due in one year, Amortized Cost	$ 2,012
Due after one year through five years, Amortized Cost	1,193
Due after five years through ten years, Amortized Cost	33,218
Due after ten years, Amortized Cost	44,470
Totals, Amortized Cost	80,893
Due in one year, Amortized Cost, Held to Maturity
Due after one year through five years, Amortized Cost, Held to Maturity
Due after five years through ten years, Amortized Cost, Held to Maturity
Due after ten years, Amortized Cost, Held to Maturity	443,509
Totals, Amortized Cost, Held to Maturity	443,509	240,504	0
Due in one year, Estimated Fair Value	2,044
Due after one year through five years, Estimated Fair Value	1,244
Due after five years through ten years, Estimated Fair Value	34,556
Due after ten years, Estimated Fair Value	47,118
Totals, Estimated Fair Value	84,962
Due in one year, Estimated Fair Value, Held to Maturity
Due after one year through five years, Estimated Fair Value, Held to Maturity
Due after five years through ten years, Estimated Fair Value, Held to Maturity
Due after ten years, Estimated Fair Value, Held to Maturity	446,804
Estimated Fair Value, Held to Maturity	$ 446,804	$ 233,807