Investments (Amortized Cost and Estimated Fair Values of investment securities Available-For-Sale by Contractual Maturity) (Detail) (USD $)	Dec. 31, 2014	Dec. 31, 2013
Amortized Cost
Due in one year or less	$ 691,000	$ 2,080,000
Due after one year through five years	47,900,000	12,766,000
Due after five years through ten years	332,841,000	392,389,000
Due after ten years	276,647,000	342,732,000
Total debt securities available for sale	658,079,000	749,967,000
Estimated Fair Value
Due in one year or less	714,000	2,085,000
Due after one year through five years	49,385,000	13,285,000
Due after five years through ten years	340,852,000	392,339,000
Due after ten years	280,535,000	342,852,000
Estimated Fair Value	$ 671,486,000	$ 750,561,000