Investments in debt and equity securities available-for-sale - Gross Unrealized Gains and Losses (Detail) - USD ($) $ in Thousands	Mar. 31, 2016	Dec. 31, 2015
Schedule of Available-for-sale Securities [Line Items]
Gains	$ 25,062	$ 18,179
Losses	1,789	3,714
Municipal
Schedule of Available-for-sale Securities [Line Items]
Gains	2,399	1,720
Losses	1	12
Corporate
Schedule of Available-for-sale Securities [Line Items]
Gains	12,128	7,700
Losses	255	2,109
Foreign
Schedule of Available-for-sale Securities [Line Items]
Gains	4,780	3,789
Losses	171	482
U.S. Treasury Bonds
Schedule of Available-for-sale Securities [Line Items]
Gains	95	128
Losses	2	25
Equity securities
Schedule of Available-for-sale Securities [Line Items]
Gains	5,660	4,842
Losses	$ 1,360	$ 1,086