Amortized Cost and Fair Value of Available for Sale Investment Securities (Detail) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2013		Dec. 31, 2012
Schedule of Available-for-sale Securities [Line Items]
Due in one year or less, Amortized Cost	$ 9,046		$ 10,571
Due after one year through five years, Amortized Cost	53,240		38,056
Due after five years through ten years, Amortized Cost	59,537		40,635
Due after ten years, Amortized Cost	14,291		18,415
Mortgage-related securities, Amortized Cost	170,573	[1]	190,653	[1]
Total maturing investments, Amortized Cost	306,687		298,330
Bond mutual funds and other non-maturity investments, Amortized Cost	13,343		13,417
Total	320,030		311,747
Due in one year or less, Fair Value	9,049		10,590
Due after one year through five years, Fair Value	53,248		38,171
Due after five years through ten years, Fair Value	58,144		40,714
Due after ten years, Fair Value	14,281		19,044
Mortgage-related securities, Fair Value	171,606	[1]	194,529	[1]
Total maturing investments, Fair Value	306,328		303,048
Bond mutual funds and other non-maturity investments, Fair Value	13,589		13,566
Total	$ 319,917		$ 316,614

[1] Expected maturities of mortgage-related securities may differ from contractual maturities as borrowers may have the right to call or prepay obligations with or without call or prepayment penalties.