Investments in debt and equity securities - Gross Unrealized Losses on Investments and Fair Values of Related Securities (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Schedule of Available-for-sale Securities [Line Items]
Losses, Less than 12 months	$ 575	$ 4,062
Fair Values, Less than 12 months	50,616	162,540
Losses, More than 12 months	550	2,192
Fair Values, More than 12 months	48,761	76,145
Total Losses	1,125	6,254
Total Fair Values	99,377	238,685
Municipal
Schedule of Available-for-sale Securities [Line Items]
Losses, Less than 12 months	1	770
Fair Values, Less than 12 months	2,673	27,686
Losses, More than 12 months	91	43
Fair Values, More than 12 months	12,076	628
Total Losses	92	813
Total Fair Values	14,749	28,314
Corporate
Schedule of Available-for-sale Securities [Line Items]
Losses, Less than 12 months	107	1,682
Fair Values, Less than 12 months	7,167	66,776
Losses, More than 12 months	339	1,546
Fair Values, More than 12 months	26,545	21,710
Total Losses	446	3,228
Total Fair Values	33,712	88,486
Foreign
Schedule of Available-for-sale Securities [Line Items]
Losses, Less than 12 months	11	1,539
Fair Values, Less than 12 months	34,236	63,039
Losses, More than 12 months	93	603
Fair Values, More than 12 months	6,446	53,807
Total Losses	104	2,142
Total Fair Values	40,682	116,846
U.S. Treasury Bonds
Schedule of Available-for-sale Securities [Line Items]
Losses, Less than 12 months	0	60
Fair Values, Less than 12 months	0	3,772
Losses, More than 12 months	27	0
Fair Values, More than 12 months	3,694	0
Total Losses	27	60
Total Fair Values	3,694	3,772
Equity securities
Schedule of Available-for-sale Securities [Line Items]
Losses, Less than 12 months	456	11
Fair Values, Less than 12 months	6,540	1,267
Losses, More than 12 months	0	0
Fair Values, More than 12 months	0	0
Total Losses	456	11
Total Fair Values	$ 6,540	$ 1,267