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	Sep. 30, 2013	Dec. 31, 2012
Investment [Line Items]
Amortized Cost	$ 550,153	$ 583,287
Gross Unrealized Gains	28,388	44,361
Gross Unrealized Losses	(1,552)	(299)
Fair Value	576,989	627,349
States and political subdivisions [Member]
Investment [Line Items]
Amortized Cost	388,694	400,687
Gross Unrealized Gains	20,250	31,387
Gross Unrealized Losses	(1,362)	(106)
Fair Value	407,582	431,968
Corporate bonds [Member]
Investment [Line Items]
Amortized Cost	71,126	82,824
Gross Unrealized Gains	965	1,565
Gross Unrealized Losses	(19)	(12)
Fair Value	72,072	84,377
Commercial mortgage-backed securities [Member]
Investment [Line Items]
Amortized Cost	51,414	51,529
Gross Unrealized Gains	3,861	6,433
Gross Unrealized Losses
Fair Value	55,275	57,962
U.S. agency-based mortgage-backed securities [Member]
Investment [Line Items]
Amortized Cost	23,876	32,984
Gross Unrealized Gains	1,966	3,063
Gross Unrealized Losses		(5)
Fair Value	25,842	36,042
U.S. Treasury securities and obligations of U.S. government agencies [Member]
Investment [Line Items]
Amortized Cost	11,522	11,034
Gross Unrealized Gains	1,130	1,721
Gross Unrealized Losses
Fair Value	12,652	12,755
Asset-backed securities [Member]
Investment [Line Items]
Amortized Cost	3,521	4,229
Gross Unrealized Gains	216	192
Gross Unrealized Losses	(171)	(176)
Fair Value	$ 3,566	$ 4,245