Investments in debt and equity securities available-for-sale - Gross Unrealized Losses on Investments and Fair Values of Related Securities (Detail) - USD ($) $ in Thousands	Jun. 30, 2015	Dec. 31, 2014
Schedule of Available-for-sale Securities [Line Items]
Losses, Less than 12 months	$ 2,790	$ 575
Fair Values, Less than 12 months	52,317	50,616
Losses, More than 12 months	1,047	550
Fair Values, More than 12 months	51,693	48,761
Total Losses	3,837	1,125
Total Fair Values	104,010	99,377
Municipal
Schedule of Available-for-sale Securities [Line Items]
Losses, Less than 12 months	247	1
Fair Values, Less than 12 months	12,126	2,673
Losses, More than 12 months	58	91
Fair Values, More than 12 months	11,430	12,076
Total Losses	305	92
Total Fair Values	23,556	14,749
Corporate
Schedule of Available-for-sale Securities [Line Items]
Losses, Less than 12 months	1,472	107
Fair Values, Less than 12 months	22,688	7,167
Losses, More than 12 months	833	339
Fair Values, More than 12 months	31,840	26,545
Total Losses	2,305	446
Total Fair Values	54,528	33,712
Foreign
Schedule of Available-for-sale Securities [Line Items]
Losses, Less than 12 months	106	11
Fair Values, Less than 12 months	4,670	34,236
Losses, More than 12 months	154	93
Fair Values, More than 12 months	4,826	6,446
Total Losses	260	104
Total Fair Values	9,496	40,682
U.S. Treasury Bonds
Schedule of Available-for-sale Securities [Line Items]
Losses, Less than 12 months	4	0
Fair Values, Less than 12 months	1,196	0
Losses, More than 12 months	2	27
Fair Values, More than 12 months	3,597	3,694
Total Losses	6	27
Total Fair Values	4,793	3,694
Equity securities
Schedule of Available-for-sale Securities [Line Items]
Losses, Less than 12 months	961	456
Fair Values, Less than 12 months	11,637	6,540
Losses, More than 12 months	0	0
Fair Values, More than 12 months	0	0
Total Losses	961	456
Total Fair Values	$ 11,637	$ 6,540