Investments in debt and equity securities - Gross Unrealized Gains and Losses (Detail) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Schedule of Available-for-sale Securities [Line Items]
Gross unrealized gains	$ 18,179	$ 23,848
Gross unrealized losses	3,714	1,125
Municipal
Schedule of Available-for-sale Securities [Line Items]
Gross unrealized gains	1,720	1,125
Gross unrealized losses	12	92
Corporate
Schedule of Available-for-sale Securities [Line Items]
Gross unrealized gains	7,700	12,559
Gross unrealized losses	2,109	446
Foreign
Schedule of Available-for-sale Securities [Line Items]
Gross unrealized gains	3,789	3,690
Gross unrealized losses	482	104
U.S. Treasury Bonds
Schedule of Available-for-sale Securities [Line Items]
Gross unrealized gains	128	492
Gross unrealized losses	25	27
Equity securities
Schedule of Available-for-sale Securities [Line Items]
Gross unrealized gains	4,842	5,982
Gross unrealized losses	$ 1,086	$ 456