Investments in debt and equity securities - Gross Unrealized Losses on Investments and Fair Values of Related Securities (Detail) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Losses
Less than 12 months	$ 4,801	$ 2,518
More than 12 months	440	1,196
Total	5,241	3,714
Fair values
Less than 12 months	239,812	126,438
More than 12 months	5,410	44,448
Total	245,222	170,886
Municipal
Losses
Less than 12 months	575	9
More than 12 months	0	3
Total	575	12
Fair values
Less than 12 months	32,038	2,230
More than 12 months	0	1,615
Total	32,038	3,845
Corporate
Losses
Less than 12 months	2,189	1,461
More than 12 months	0	648
Total	2,189	2,109
Fair values
Less than 12 months	119,965	83,565
More than 12 months	0	32,871
Total	119,965	116,436
Foreign
Losses
Less than 12 months	1,427	322
More than 12 months	193	160
Total	1,620	482
Fair values
Less than 12 months	70,012	35,008
More than 12 months	3,160	3,155
Total	73,172	38,163
U.S. Treasury Bonds
Losses
Less than 12 months	186	6
More than 12 months	0	19
Total	186	25
Fair values
Less than 12 months	11,847	1,195
More than 12 months	0	3,583
Total	11,847	4,778
Equity securities
Losses
Less than 12 months	424	720
More than 12 months	247	366
Total	671	1,086
Fair values
Less than 12 months	5,950	4,440
More than 12 months	2,250	3,224
Total	$ 8,200	$ 7,664