Investment Securities - Schedule of Amortized Cost and Estimated Fair Value of Debt Securities (Detail) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2013
Schedule of Available-for-sale Securities [Line Items]
Due in one year or less, Amortized Cost	$ 62,900
Due after one year through three years, Amortized Cost	186,188
Due after three years through five years, Amortized Cost	63,464
Due after five years, Amortized Cost	45,056
Total amortized cost of debt securities	357,608
Due in one year or less, Estimated Fair Value	63,031
Due after one year through three years, Estimated Fair Value	187,252
Due after three years through five years, Estimated Fair Value	63,781
Due after five years, Estimated Fair Value	45,910
Total estimated fair value of debt securities	$ 359,974