Investments (Amortized Cost and Estimated Fair Values of investment securities Available-For-Sale by Contractual Maturity) (Detail) (USD $)	Mar. 31, 2015	Dec. 31, 2014
Amortized Cost
Due in one year or less	$ 691,000	$ 691,000
Due after one year through five years	73,069,000	47,900,000
Due after five years through ten years	304,330,000	332,841,000
Due after ten years	260,855,000	276,647,000
Total debt securities available for sale	638,945,000	658,079,000
Estimated Fair Value
Due in one year or less	708,000	714,000
Due after one year through five years	75,408,000	49,385,000
Due after five years through ten years	314,650,000	340,852,000
Due after ten years	266,220,000	280,535,000
Estimated Fair Value	$ 656,986,000	$ 671,486,000