Investments in debt and equity securities - Gross Unrealized Losses on Investments and Fair Values of Related Securities (Detail) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Schedule of Available-for-sale Securities [Line Items]
Losses, Less than 12 months	$ 2,518	$ 575
Fair Values, Less than 12 months	126,438	50,616
Losses, More than 12 months	1,196	550
Fair Values, More than 12 months	44,448	48,761
Total Losses	3,714	1,125
Total Fair Values	170,886	99,377
Municipal
Schedule of Available-for-sale Securities [Line Items]
Losses, Less than 12 months	9	1
Fair Values, Less than 12 months	2,230	2,673
Losses, More than 12 months	3	91
Fair Values, More than 12 months	1,615	12,076
Total Losses	12	92
Total Fair Values	3,845	14,749
Corporate
Schedule of Available-for-sale Securities [Line Items]
Losses, Less than 12 months	1,461	107
Fair Values, Less than 12 months	83,565	7,167
Losses, More than 12 months	648	339
Fair Values, More than 12 months	32,871	26,545
Total Losses	2,109	446
Total Fair Values	116,436	33,712
Foreign
Schedule of Available-for-sale Securities [Line Items]
Losses, Less than 12 months	322	11
Fair Values, Less than 12 months	35,008	34,236
Losses, More than 12 months	160	93
Fair Values, More than 12 months	3,155	6,446
Total Losses	482	104
Total Fair Values	38,163	40,682
U.S. Treasury Bonds
Schedule of Available-for-sale Securities [Line Items]
Losses, Less than 12 months	6	0
Fair Values, Less than 12 months	1,195	0
Losses, More than 12 months	19	27
Fair Values, More than 12 months	3,583	3,694
Total Losses	25	27
Total Fair Values	4,778	3,694
Equity securities
Schedule of Available-for-sale Securities [Line Items]
Losses, Less than 12 months	720	456
Fair Values, Less than 12 months	4,440	6,540
Losses, More than 12 months	366	0
Fair Values, More than 12 months	3,224	0
Total Losses	1,086	456
Total Fair Values	$ 7,664	$ 6,540