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, 2016	Dec. 31, 2015
Schedule of Available-for-sale Securities [Line Items]
Losses, Less than 12 months	$ 506	$ 2,518
Fair values, Less than 12 months	39,119	126,438
Losses, More than 12 months	330	1,196
Fair values, More than 12 months	2,217	44,448
Total Losses	836	3,714
Total Fair Values	41,336	170,886
Municipal
Schedule of Available-for-sale Securities [Line Items]
Losses, Less than 12 months		9
Fair values, Less than 12 months		2,230
Losses, More than 12 months		3
Fair values, More than 12 months		1,615
Total Losses		12
Total Fair Values		3,845
Corporate
Schedule of Available-for-sale Securities [Line Items]
Losses, Less than 12 months	10	1,461
Fair values, Less than 12 months	3,901	83,565
Losses, More than 12 months	0	648
Fair values, More than 12 months	0	32,871
Total Losses	10	2,109
Total Fair Values	3,901	116,436
Foreign
Schedule of Available-for-sale Securities [Line Items]
Losses, Less than 12 months	42	322
Fair values, Less than 12 months	32,547	35,008
Losses, More than 12 months	0	160
Fair values, More than 12 months	0	3,155
Total Losses	42	482
Total Fair Values	32,547	38,163
U.S. Treasury Bonds
Schedule of Available-for-sale Securities [Line Items]
Losses, Less than 12 months		6
Fair values, Less than 12 months		1,195
Losses, More than 12 months		19
Fair values, More than 12 months		3,583
Total Losses		25
Total Fair Values		4,778
Equity securities
Schedule of Available-for-sale Securities [Line Items]
Losses, Less than 12 months	454	720
Fair values, Less than 12 months	2,671	4,440
Losses, More than 12 months	330	366
Fair values, More than 12 months	2,217	3,224
Total Losses	784	1,086
Total Fair Values	$ 4,888	$ 7,664