Investments in debt and equity securities available-for-sale - Gross Unrealized Losses on Investments and Fair Values of Related Securities (Detail) - USD ($) $ in Thousands	Mar. 31, 2016	Dec. 31, 2015
Schedule of Available-for-sale Securities [Line Items]
Losses, Less than 12 months	$ 1,413	$ 2,518
Fair Values, Less than 12 months	77,226	126,438
Losses, More than 12 months	376	1,196
Fair Values, More than 12 months	10,017	44,448
Total Losses	1,789	3,714
Total Fair Values	87,243	170,886
Municipal
Schedule of Available-for-sale Securities [Line Items]
Losses, Less than 12 months	1	9
Fair Values, Less than 12 months	1,256	2,230
Losses, More than 12 months	0	3
Fair Values, More than 12 months	0	1,615
Total Losses	1	12
Total Fair Values	1,256	3,845
Corporate
Schedule of Available-for-sale Securities [Line Items]
Losses, Less than 12 months	225	1,461
Fair Values, Less than 12 months	30,154	83,565
Losses, More than 12 months	30	648
Fair Values, More than 12 months	5,020	32,871
Total Losses	255	2,109
Total Fair Values	35,174	116,436
Foreign
Schedule of Available-for-sale Securities [Line Items]
Losses, Less than 12 months	65	322
Fair Values, Less than 12 months	34,396	35,008
Losses, More than 12 months	106	160
Fair Values, More than 12 months	3,976	3,155
Total Losses	171	482
Total Fair Values	38,372	38,163
U.S. Treasury Bonds
Schedule of Available-for-sale Securities [Line Items]
Losses, Less than 12 months	2	6
Fair Values, Less than 12 months	3,601	1,195
Losses, More than 12 months	0	19
Fair Values, More than 12 months	0	3,583
Total Losses	2	25
Total Fair Values	3,601	4,778
Equity securities
Schedule of Available-for-sale Securities [Line Items]
Losses, Less than 12 months	1,120	720
Fair Values, Less than 12 months	7,819	4,440
Losses, More than 12 months	240	366
Fair Values, More than 12 months	1,021	3,224
Total Losses	1,360	1,086
Total Fair Values	$ 8,840	$ 7,664