Investments in debt and equity securities available-for-sale - Gross Unrealized Losses on Investments and Fair Values of Related Securities (Detail) - USD ($) $ in Thousands	Sep. 30, 2015	Dec. 31, 2014
Schedule of Available-for-sale Securities [Line Items]
Losses, Less than 12 months	$ 4,892	$ 575
Fair Values, Less than 12 months	83,285	50,616
Losses, More than 12 months	765	550
Fair Values, More than 12 months	34,900	48,761
Total Losses	5,657	1,125
Total Fair Values	118,185	99,377
Municipal
Schedule of Available-for-sale Securities [Line Items]
Losses, Less than 12 months	56	1
Fair Values, Less than 12 months	4,987	2,673
Losses, More than 12 months	0	91
Fair Values, More than 12 months	0	12,076
Total Losses	56	92
Total Fair Values	4,987	14,749
Corporate
Schedule of Available-for-sale Securities [Line Items]
Losses, Less than 12 months	646	107
Fair Values, Less than 12 months	30,408	7,167
Losses, More than 12 months	590	339
Fair Values, More than 12 months	30,043	26,545
Total Losses	1,236	446
Total Fair Values	60,451	33,712
Foreign
Schedule of Available-for-sale Securities [Line Items]
Losses, Less than 12 months	171	11
Fair Values, Less than 12 months	33,756	34,236
Losses, More than 12 months	171	93
Fair Values, More than 12 months	4,634	6,446
Total Losses	342	104
Total Fair Values	38,390	40,682
U.S. Treasury Bonds
Schedule of Available-for-sale Securities [Line Items]
Losses, Less than 12 months		0
Fair Values, Less than 12 months		0
Losses, More than 12 months		27
Fair Values, More than 12 months		3,694
Total Losses		27
Total Fair Values		3,694
Equity securities
Schedule of Available-for-sale Securities [Line Items]
Losses, Less than 12 months	4,019	456
Fair Values, Less than 12 months	14,134	6,540
Losses, More than 12 months	4	0
Fair Values, More than 12 months	223	0
Total Losses	4,023	456
Total Fair Values	$ 14,357	$ 6,540