Investments in debt and equity securities - Gross Unrealized Gains and Losses (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Schedule of Available-for-sale Securities [Line Items]
Gross unrealized gains	$ 23,848	$ 14,764
Gross unrealized losses	1,125	6,254
Municipal
Schedule of Available-for-sale Securities [Line Items]
Gross unrealized gains	1,125	258
Gross unrealized losses	92	813
Corporate
Schedule of Available-for-sale Securities [Line Items]
Gross unrealized gains	12,559	9,955
Gross unrealized losses	446	3,228
Foreign
Schedule of Available-for-sale Securities [Line Items]
Gross unrealized gains	3,690	363
Gross unrealized losses	104	2,142
U.S. Treasury Bonds
Schedule of Available-for-sale Securities [Line Items]
Gross unrealized gains	492	924
Gross unrealized losses	27	60
Equity securities
Schedule of Available-for-sale Securities [Line Items]
Gross unrealized gains	5,982	3,264
Gross unrealized losses	$ 456	$ 11