Investments (Amortized Cost and Estimated Fair Values of Debt Securities Held-To-Maturity by Contractual Maturity) (Detail) (USD $)	Mar. 31, 2015	Dec. 31, 2014
Amortized Cost
Due in one year or less	$ 0	$ 1,690,000
Due after one year through five years	6,980,000	6,763,000
Due after five years through ten years	164,343,000	163,252,000
Due after ten years	46,234,000	48,268,000
Total debt securities held-to-maturity	217,557,000	219,973,000
Estimated Fair Value
Due in one year or less	0	1,694,000
Due after one year through five years	7,161,000	6,938,000
Due after five years through ten years	167,489,000	164,787,000
Due after ten years	47,037,000	48,841,000
Total debt securities held-to-maturity	$ 221,687,000	$ 222,260,000