Investments - Gross Unrealized Gains and Losses and Cost and Fair Value of Investments Classified as Held-to-maturity (Detail) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012
Investment [Line Items]
Cost or Amortized Cost	$ 565,874	$ 583,287
Gross Unrealized Gains	30,173	44,361
Gross Unrealized Losses	(1,516)	(299)
Fair Value	594,531	627,349
States and political subdivisions [Member]
Investment [Line Items]
Cost or Amortized Cost	395,641	400,687
Gross Unrealized Gains	21,298	31,387
Gross Unrealized Losses	(1,262)	(106)
Fair Value	415,677	431,968
Corporate bonds [Member]
Investment [Line Items]
Cost or Amortized Cost	77,235	82,824
Gross Unrealized Gains	1,045	1,565
Gross Unrealized Losses	(74)	(12)
Fair Value	78,206	84,377
Commercial mortgage-backed securities [Member]
Investment [Line Items]
Cost or Amortized Cost	51,502	51,529
Gross Unrealized Gains	4,230	6,433
Fair Value	55,732	57,962
U.S. agency-based mortgage-backed securities [Member]
Investment [Line Items]
Cost or Amortized Cost	26,732	32,984
Gross Unrealized Gains	2,179	3,063
Gross Unrealized Losses		(5)
Fair Value	28,911	36,042
U.S. Treasury securities and obligations of U.S. government agencies [Member]
Investment [Line Items]
Cost or Amortized Cost	11,020	11,034
Gross Unrealized Gains	1,205	1,721
Fair Value	12,225	12,755
Asset-backed securities [Member]
Investment [Line Items]
Cost or Amortized Cost	3,744	4,229
Gross Unrealized Gains	216	192
Gross Unrealized Losses	(180)	(176)
Fair Value	$ 3,780	$ 4,245