Investments in debt and equity securities available-for-sale - Gross Unrealized Gains and Losses (Detail) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Schedule of Available-for-sale Securities [Line Items]
Gross unrealized gains	$ 27,142	$ 23,848
Gross unrealized losses	1,176	1,125
Municipal
Schedule of Available-for-sale Securities [Line Items]
Gross unrealized gains	1,318	1,125
Gross unrealized losses	49	92
Corporate
Schedule of Available-for-sale Securities [Line Items]
Gross unrealized gains	15,042	12,559
Gross unrealized losses	152	446
Foreign
Schedule of Available-for-sale Securities [Line Items]
Gross unrealized gains	5,798	3,690
Gross unrealized losses	126	104
U.S. Treasury Bonds
Schedule of Available-for-sale Securities [Line Items]
Gross unrealized gains	414	492
Gross unrealized losses	2	27
Equity securities
Schedule of Available-for-sale Securities [Line Items]
Gross unrealized gains	4,570	5,982
Gross unrealized losses	$ 847	$ 456