Investments - Gross Unrealized Gains and Losses and the Amortized Cost and Fair Value of Investments Classified as Held-to-maturity (Detail) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2014	Dec. 31, 2013
Investment [Line Items]
Amortized Cost	$ 548,612	$ 536,583
Gross Unrealized Gains	27,079	25,934
Gross Unrealized Losses	(516)	(1,338)
Fair Value	575,175	561,179
States and political subdivisions [Member]
Investment [Line Items]
Amortized Cost	368,697	381,674
Gross Unrealized Gains	19,970	18,634
Gross Unrealized Losses	(303)	(1,153)
Fair Value	388,364	399,155
Corporate bonds [Member]
Investment [Line Items]
Amortized Cost	94,099	67,423
Gross Unrealized Gains	945	861
Gross Unrealized Losses	(74)	(41)
Fair Value	94,970	68,243
Commercial mortgage-backed securities [Member]
Investment [Line Items]
Amortized Cost	49,884	50,813
Gross Unrealized Gains	3,079	3,431
Fair Value	52,963	54,244
U.S. agency-based mortgage-backed securities [Member]
Investment [Line Items]
Amortized Cost	20,418	21,775
Gross Unrealized Gains	1,794	1,790
Fair Value	22,212	23,565
U.S. Treasury securities and obligations of U.S. government agencies [Member]
Investment [Line Items]
Amortized Cost	12,265	11,514
Gross Unrealized Gains	1,077	1,002
Gross Unrealized Losses	(6)
Fair Value	13,336	12,516
Asset-backed securities [Member]
Investment [Line Items]
Amortized Cost	3,249	3,384
Gross Unrealized Gains	214	216
Gross Unrealized Losses	(133)	(144)
Fair Value	$ 3,330	$ 3,456