Investments - Gross Unrealized Gains and Losses and Amortized Cost and Fair Value of Investments Classified as Held-to-maturity (Detail) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2014	Dec. 31, 2013
Investment [Line Items]
Amortized Cost	$ 579,145	$ 536,583
Gross Unrealized Gains	28,611	25,934
Gross Unrealized Losses	(244)	(1,338)
Fair Value	607,512	561,179
States and political subdivisions [Member]
Investment [Line Items]
Amortized Cost	392,381	381,674
Gross Unrealized Gains	21,821	18,634
Gross Unrealized Losses	(66)	(1,153)
Fair Value	414,136	399,155
Corporate bonds [Member]
Investment [Line Items]
Amortized Cost	102,925	67,423
Gross Unrealized Gains	756	861
Gross Unrealized Losses	(57)	(41)
Fair Value	103,624	68,243
Commercial mortgage-backed securities [Member]
Investment [Line Items]
Amortized Cost	49,327	50,813
Gross Unrealized Gains	2,767	3,431
Fair Value	52,094	54,244
U.S. agency-based mortgage-backed securities [Member]
Investment [Line Items]
Amortized Cost	19,170	21,775
Gross Unrealized Gains	1,892	1,790
Fair Value	21,062	23,565
U.S. Treasury securities and obligations of U.S. government agencies [Member]
Investment [Line Items]
Amortized Cost	12,259	11,514
Gross Unrealized Gains	1,132	1,002
Gross Unrealized Losses	(1)
Fair Value	13,390	12,516
Asset-backed securities [Member]
Investment [Line Items]
Amortized Cost	3,083	3,384
Gross Unrealized Gains	243	216
Gross Unrealized Losses	(120)	(144)
Fair Value	$ 3,206	$ 3,456