Investments (Gross Unrealized Losses and Fair Value by Length of Time for Held-To-Maturity Securities) (Detail) (Held-to-Maturity Securities, USD $)	Dec. 31, 2014 Securities	Dec. 31, 2013 Securities
Schedule Of Held To Maturity Securities [Line Items]
Number of securities	49	121
Fair Value	$ 94,805,000	$ 153,921,000
Less than 12 months	18,000	10,193,000
More than 12 months	2,041,000	387,000
Total Unrealized Losses	2,059,000	10,580,000
U.S. government agencies
Schedule Of Held To Maturity Securities [Line Items]
Number of securities	8	8
Fair Value	62,778,000	59,678,000
Less than 12 months	0	4,827,000
More than 12 months	1,734,000	0
Total Unrealized Losses	1,734,000	4,827,000
State and municipal
Schedule Of Held To Maturity Securities [Line Items]
Number of securities	41	113
Fair Value	32,027,000	94,243,000
Less than 12 months	18,000	5,366,000
More than 12 months	307,000	387,000
Total Unrealized Losses	$ 325,000	$ 5,753,000