Investments in debt and equity securities available-for-sale - Gross Unrealized Losses on Investments and Fair Values of Related Securities (Detail) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Schedule of Available-for-sale Securities [Line Items]
Losses, Less than 12 months	$ 931	$ 575
Fair Values, Less than 12 months	22,225	50,616
Losses, More than 12 months	245	550
Fair Values, More than 12 months	19,464	48,761
Total Losses	1,176	1,125
Total Fair Values	41,689	99,377
Municipal
Schedule of Available-for-sale Securities [Line Items]
Losses, Less than 12 months	42	1
Fair Values, Less than 12 months	5,317	2,673
Losses, More than 12 months	7	91
Fair Values, More than 12 months	3,394	12,076
Total Losses	49	92
Total Fair Values	8,711	14,749
Corporate
Schedule of Available-for-sale Securities [Line Items]
Losses, Less than 12 months	36	107
Fair Values, Less than 12 months	6,356	7,167
Losses, More than 12 months	116	339
Fair Values, More than 12 months	7,893	26,545
Total Losses	152	446
Total Fair Values	14,249	33,712
Foreign
Schedule of Available-for-sale Securities [Line Items]
Losses, Less than 12 months	6	11
Fair Values, Less than 12 months	1,487	34,236
Losses, More than 12 months	120	93
Fair Values, More than 12 months	4,581	6,446
Total Losses	126	104
Total Fair Values	6,068	40,682
U.S. Treasury Bonds
Schedule of Available-for-sale Securities [Line Items]
Losses, Less than 12 months	0	0
Fair Values, Less than 12 months	0	0
Losses, More than 12 months	2	27
Fair Values, More than 12 months	3,596	3,694
Total Losses	2	27
Total Fair Values	3,596	3,694
Equity securities
Schedule of Available-for-sale Securities [Line Items]
Losses, Less than 12 months	847	456
Fair Values, Less than 12 months	9,065	6,540
Losses, More than 12 months	0	0
Fair Values, More than 12 months	0	0
Total Losses	847	456
Total Fair Values	$ 9,065	$ 6,540